Transforming growth factor-beta induces osteoclast ruffling and chemotaxis: potential role in osteoclast recruitment

Spontaneous Osteoclastogenesis, a risk factor for bone metastasis in advanced luminal A-type breast cancer patients

Introduction

Osteolytic bone metastasis in advanced breast cancer stages are a major complication for patient´s quality life and a sign of low survival prognosis. Permissive microenvironments which allow cancer cell secondary homing and later proliferation are fundamental for metastatic processes. The causes and mechanisms behind bone metastasis in breast cancer patients are still an unsolved puzzle. Therefore, in this work we contribute to describe bone marrow pre-metastatic niche in advanced breast cancer patients.

Results

We show an increase in osteoclasts precursors with a concomitant imbalance towards spontaneous osteoclastogenesis which can be evidenced at bone marrow and peripheral levels. Pro-osteoclastogenic factors RANKL and CCL-2 may contribute to bone resorption signature observed in bone marrow. Meanwhile, expression levels of specific microRNAs in primary breast tumors may already indicate a pro-osteoclastogenic scenario prior to bone metastasis.

Discussion

The discovery of prognostic biomarkers and novel therapeutic targets linked to bone metastasis initiation and development are a promising perspective for preventive treatments and metastasis management in advanced breast cancer patients.

Monocyte/Macrophage Lineage Cells From Fetal Erythromyeloid Progenitors Orchestrate Bone Remodeling and Repair

A third of the population sustains a bone fracture, and the pace of fracture healing slows with age. The slower pace of repair is responsible for the increased morbidity in older individuals who sustain a fracture. Bone healing progresses through overlapping phases, initiated by cells of the monocyte/macrophage lineage. The repair process ends with remodeling. This last phase is controlled by osteoclasts, which are bone-specific multinucleated cells also of the monocyte/macrophage lineage. The slower rate of healing in aging can be rejuvenated by macrophages from young animals, and secreted proteins from macrophage regulate undifferentiated mesenchymal cells to become bone-forming osteoblasts. Macrophages can derive from fetal erythromyeloid progenitors or from adult hematopoietic progenitors. Recent studies show that fetal erythromyeloid progenitors are responsible for the osteoclasts that form the space in bone for hematopoiesis and the fetal osteoclast precursors reside in the spleen postnatally, traveling through the blood to participate in fracture repair. Differences in secreted proteins between macrophages from old and young animals regulate the efficiency of osteoblast differentiation from undifferentiated mesenchymal precursor cells. Interestingly, during the remodeling phase osteoclasts can form from the fusion between monocyte/macrophage lineage cells from the fetal and postnatal precursor populations. Data from single cell RNA sequencing identifies specific markers for populations derived from the different precursor populations, a finding that can be used in future studies. Here, we review the diversity of macrophages and osteoclasts, and discuss recent finding about their developmental origin and functions, which provides novel insights into their roles in bone homeostasis and repair.

TGF-Beta Signaling in Bone with Chronic Kidney Disease

Transforming growth factor (TGF)-β signaling is not only important in skeletal development, but also essential in bone remodeling in adult bone. The bone remodeling process involves integrated cell activities induced by multiple stimuli to balance bone resorption and bone formation. TGF-β plays a role in bone remodeling by coordinating cell activities to maintain bone homeostasis. However, mineral metabolism disturbance in chronic kidney disease (CKD) results in abnormal bone remodeling, which leads to ectopic calcification in CKD. High circulating levels of humoral factors such as parathyroid hormone, fibroblast growth factor 23, and Wnt inhibitors modulate bone remodeling in CKD. Several reports have revealed that TGF-β is involved in the production and functions of these factors in bone. TGF-β may act as a factor that mediates abnormal bone remodeling in CKD.

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

The transforming growth factor β (TGF-β) family of signaling molecules, which includes TGF-βs, activins, inhibins, and numerous bone morphogenetic proteins (BMPs) and growth and differentiation factors (GDFs), has important functions in all cells and tissues, including soft connective tissues and the skeleton. Specific TGF-β family members play different roles in these tissues, and their activities are often balanced with those of other TGF-β family members and by interactions with other signaling pathways. Perturbations in TGF-β family pathways are associated with numerous human diseases with prominent involvement of the skeletal and cardiovascular systems. This review focuses on the role of this family of signaling molecules in the pathologies of connective tissues that manifest in rare genetic syndromes (e.g., syndromic presentations of thoracic aortic aneurysm), as well as in more common disorders (e.g., osteoarthritis and osteoporosis). Many of these diseases are caused by or result in pathological alterations of the complex relationship between the TGF-β family of signaling mediators and the extracellular matrix in connective tissues.

Stimulation of osteoclast migration and bone resorption by C-C chemokine ligands 19 and 21

Osteoclasts are responsible for the bone erosion associated with rheumatoid arthritis (RA). The upregulation of the chemokines CCL19 and CCL21 and their receptor CCR7 has been linked to RA pathogenesis. The purpose of this study was to evaluate the effects of CCL19 and CCL21 on osteoclasts and to reveal their underlying mechanisms. The expression of CCL19, CCL21 and CCR7 was higher in RA patients than in osteoarthritis patients. In differentiating osteoclasts, tumor necrosis factor-α, interleukin-1β and lipopolysaccharide stimulated CCR7 expression. CCL19 and CCL21 promoted osteoclast migration and resorption activity. These effects were dependent on the presence of CCR7 and abolished by the inhibition of the Rho signaling pathway. CCL19 and CCL21 promoted bone resorption by osteoclasts in an in vivo mice calvarial model. These findings demonstrate for the first time that CCL19, CCL21 and CCR7 play important roles in bone destruction by increasing osteoclast migration and resorption activity. This study also suggests that the interaction of CCL19 and CCL21 with CCR7 is an effective strategic focus in developing therapeutics for alleviating inflammatory bone destruction.

Transforming Growth Factor β1 (TGF-β1) in the Sera of Postmenopausal Osteoporotic Females

BACKGROUND

Postmenopausal osteoporosis is a major cause of morbidity in postmenopausal females. Transforming growth factor β1 (TGF-β1) and interleukin 18 (IL-18) play complex roles in normal bone metabolism, and in pathophysiology of postmenopausal osteoporosis.

OBJECTIVES

The aim of this study was to design an analytic cross sectional study in order to further clarify the role of TGF-β1 and IL-18 in osteoporosis of postmenopausal females.

METHODS

A cross sectional study including 65 postmenopausal osteoporotic females as cases and 69 postmenopausal females of similar age without osteoporosis as controls was conducted. Dual energy X-ray absorptiometry (DXA) was used to determine bone mass density (BMD) of participants and T-scoring was applied to establish whether the patient has osteoporosis or not. Serum TGF-β1 and IL-18 levels were measured by quantitative sandwich Enzyme linked immunosorbent assay (ELISA).

RESULTS

Serum TGF-β1 levels were significantly higher in osteoporotic postmenopausal females than non-osteoporotic individuals (23.8 vs. 15.8 ng/mL; P = 0.009). There was no difference between IL-18 levels in the sera of osteoporotic and non-osteoporotic postmenopausal females in this study. There was a positive correlation between body mass index (BMI) and serum level of TGF-β1 (P = 0.04).

CONCLUSIONS

Our study demonstrated that TGF-β1 serum levels is higher in osteoporotic postmenopausal females than non-osteoporotic ones, and probably aberrant increase in TGF-β1 in postmenopausal females can result in uncoupled bone resorption and formation, which leads to osteoporosis.

The Modulatory Effects of Mesenchymal Stem Cells on Osteoclastogenesis

The effect of mesenchymal stem cells (MSCs) on bone formation has been extensively demonstrated through several in vitro and in vivo studies. However, few studies addressed the effect of MSCs on osteoclastogenesis and bone resorption. Under physiological conditions, MSCs support osteoclastogenesis through producing the main osteoclastogenic cytokines, RANKL and M-CSF. However, during inflammation, MSCs suppress osteoclast formation and activity, partly via secretion of the key anti-osteoclastogenic factor, osteoprotegerin (OPG). In vitro, co-culture of MSCs with osteoclasts in the presence of high concentrations of osteoclast-inducing factors might reflect the in vivo inflammatory pathology and prompt MSCs to exert an osteoclastogenic suppressive effect. MSCs thus seem to have a dual effect, by stimulating or inhibiting osteoclastogenesis, depending on the inflammatory milieu. This effect of MSCs on osteoclast formation seems to mirror the effect of MSCs on other immune cells, and may be exploited for the therapeutic potential of MSCs in bone loss associated inflammatory diseases.

Activin A inhibits RANKL-mediated osteoclast formation, movement and function in murine bone marrow macrophage cultures

The process of osteoclastic bone resorption is complex and regulated at multiple levels. The role of osteoclast (OCL) fusion and motility in bone resorption are unclear, with the movement of OCL on bone largely unexplored. RANKL (also known as TNFSF11) is a potent stimulator of murine osteoclastogenesis, and activin A (ActA) enhances that stimulation in whole bone marrow. ActA treatment does not induce osteoclastogenesis in stroma-free murine bone marrow macrophage cultures (BMM), but rather inhibits RANKL-induced osteoclastogenesis. We hypothesized that ActA and RANKL differentially regulate osteoclastogenesis by modulating OCL precursors and mature OCL migration. Time-lapse video microscopy measured ActA and RANKL effects on BMM and OCL motility and function. ActA completely inhibited RANKL-stimulated OCL motility, differentiation and bone resorption, through a mechanism mediated by ActA-dependent changes in SMAD2, AKT1 and inhibitor of nuclear factor κB (IκB) signaling. The potent and dominant inhibitory effect of ActA was associated with decreased OCL lifespan because ActA significantly increased activated caspase-3 in mature OCL and OCL precursors. Collectively, these data demonstrate a dual action for ActA on murine OCLs.

Osteoimmunology in orthodontic tooth movement

The skeletal and immune systems share a multitude of regulatory molecules, including cytokines, receptors, signaling molecules, and signaling transducers, thereby mutually influencing each other. In recent years, several novel insights have been attained that have enhanced our current understanding of the detailed mechanisms of osteoimmunology. In orthodontic tooth movement, immune responses mediated by periodontal tissue under mechanical force induce the generation of inflammatory responses with consequent alveolar bone resorption, and many regulators are involved in this process. In this review, we take a closer look at the cellular/molecular mechanisms and signaling involved in osteoimmunology and at relevant research progress in the context of the field of orthodontic tooth movement.

Subcellular elevation of cytosolic free calcium is required for osteoclast migration

Osteoclasts are multinucleated cells responsible for the resorption of bone and other mineralized tissues during development, physiological remodeling, and pathological bone loss. Osteoclasts have the ability to resorb substrate while concurrently migrating. However, the subcellular processes underlying migration are not well understood. It has been proposed that, in other cell types, cytosolic free Ca(2+) concentration ([Ca(2+) ]i ) regulates cell protrusion as well as retraction. Integration of these distinct events would require precise spatiotemporal patterning of subcellular Ca(2+) . The large size of osteoclasts offers a unique opportunity to monitor patterns of Ca(2+) during cell migration. We used ratiometric imaging to map [Ca(2+) ]i within rat and mouse osteoclasts. Migration was characterized by lamellipodial outgrowth at the leading edge, along with intermittent retraction of the uropod. Migrating osteoclasts displayed elevation of [Ca(2+) ]i in the uropod, that began prior to retraction. Dissipation of this [Ca(2+) ]i gradient by loading osteoclasts with the Ca(2+) chelator BAPTA abolished uropod retraction, on both glass and mineralized substrates. In contrast, elevation of [Ca(2+) ]i using ionomycin initiated prompt uropod retraction. To investigate downstream effectors, we treated cells with calpain inhibitor-1, which impaired uropod retraction. In contrast, lamellipodial outgrowth at the leading edge of osteoclasts was unaffected by any of these interventions, indicating that the signals regulating outgrowth are distinct from those triggering retraction. The large size of mature, multinucleated osteoclasts allowed us to discern a novel spatiotemporal pattern of Ca(2+) involved in cell migration. Whereas localized elevation of Ca(2+) is necessary for uropod retraction, lamellipod outgrowth is independent of Ca(2+) -a heretofore unrecognized degree of specificity underlying the regulation of osteoclast migration.

Frequencies of interleukin-6, GST and progesterone receptor gene polymorphisms in postmenopausal women with low bone mineral density

CONTEXT AND OBJECTIVE

Osteoporosis is a skeletal disorder characterized by low bone mineral density (BMD). Studies have shown that some of the genetic components relating to lower BMD may be detected by polymorphisms. Our aim was to evaluate the frequencies of interleukin-6, GST and progesterone receptor gene polymorphisms in postmenopausal women with low BMD.

DESIGN AND SETTING

Cross-sectional study, conducted in a public university in São Paulo, Brazil.

METHODS

We evaluated interleukin-6 (IL-6), progesterone receptor gene (PROGINS) and glutathione S-transferase (GST) polymorphisms in 110 postmenopausal women with no previous use of hormone therapy. Tests were performed using DNA-PCR, from oral scrapings. We used Student's t-test and a logistic regression model for statistical analysis.

RESULTS

Regarding IL-6 polymorphism, 58.2% of the patients were homozygotes (GG) and 41.8% had allele C (heterozygote or mutant homozygote + GC or CC). PROGINS genotype polymorphism was absent in 79% (wild homozygote or P1/P1) and present in 20.9% (heterozygote or P1/P2). Regarding GSTM1 polymorphism, the allele (1/1) was present in 72.7% of the patients and was absent in 27.3%. We found that IL-6 polymorphism had statistically significant correlations with the L2-L4 T-score (P = 0.032) and with BMD (P = 0.005). Women with IL-6 polymorphism were 2.3 times more likely to have a L2-L4 T-score of less than -1, compared with those not presenting this polymorphism.

CONCLUSION

IL-6 gene polymorphism was correlated with low BMD, whereas the PROGINS and GSTM1 polymorphisms did not show any correlation.

TGF-beta1 on osteoimmunology and the bone component cells

TGF-β1 is an immunoregulatory cytokine that regulates immune cell proliferation, survival, differentiation, and migration. Compelling evidence has demonstrated a strong association between the immune and skeletal systems (so called Osteoimmunology), such as the critical role of TGF-β1 in the development and maintenance of the skeletal tissue. This review provides an overview of the mechanisms in which TGF-β1 interacts with bone component cells, such as osteoblasts, osteoclasts, chondrocytes, mesenchymal stem cells, and hematopoietic stem cells, in concert with other cytokines and hormones.

Regulation of postnatal bone homeostasis by TGFβ

Perhaps more so than any other tissue, bone has pivotal mechanical and biological functions. Underlying the ability of bone to execute these functions, whether providing structural support or preserving mineral homeostasis, is the dynamic remodeling of bone matrix. Cells within bone integrate multiple stimuli to balance the deposition and resorption of bone matrix. Transforming growth factor-β (TGFβ) uniquely coordinates bone cell activity to maintain bone homeostasis. TGFβ regulates the differentiation and function of both osteoblasts and osteoclasts, from lineage recruitment to terminal differentiation, to balance bone formation and resorption. TGFβ calibrates the synthesis and material quality of bone matrix and bone's responsiveness to applied mechanical loads. Therefore, by coupling the activity of bone forming and resorbing cells, and by sensing, responding to and defining physical cues, TGFβ integrates physical and biochemical stimuli to maintain bone homeostasis. Disruption of TGFβ signaling has significant consequences on bone mass and quality. Alternatively, TGFβ is a powerful lever that has the potential to yield therapeutic benefit in cases where bone homeostasis needs to be recalibrated.

Biochemical markers of bone metabolism in gingival crevicular fluid during early orthodontic tooth movement

OBJECTIVE

To evaluate the expression of an activator of nuclear factor-kappa (RANK), osteoprotegerin (OPG), osteopontin (OPN), and transforming growth factor ß1 (TGF-ß1) in gingival crevicular fluid (GCF) of teeth subjected to orthodontic forces.

MATERIALS AND METHODS

A randomized, pilot clinical trial including 10 healthy volunteers was conducted using a split-mouth design. Orthodontic elastic separators were placed between the second premolar and first molar, with the contralateral quadrant serving as a control. The GCF samples were collected from the tension and compression sites at baseline, 24 hours, and 7 days after the placement of separators. The GCF sample volumes were measured using a Periotron 8000, and total protein concentrations were determined. Levels of RANK, OPG, OPN, and TGF-ß1 were also analyzed using a multiplex enzyme-linked immunosorbent assay.

RESULTS

The control sites remained unchanged throughout the study. In contrast, the concentration of OPG significantly decreased at the compression site by 24 hours, and the amount and concentration of RANK differed significantly between the control, compression, and tension sites after 7 days. A significant increase in absolute TGF-ß1 levels was also detected at the compression site versus the control and tension sites after 7 days.

CONCLUSION

Bone metabolism is affected by application of force to the teeth by elastic separators. Both increased expression of bone resorptive mediators (eg, RANK and TGF-ß1) and decreased expression of a bone-forming mediator (eg, OPG) on the compression side were detected.

Effect of ionic products of dicalcium silicate coating on osteoblast differentiation and collagen production via TGF-β1 pathway

In this work, the medium containing ionic products of dicalcium silicates (Ca(2)SiO(4)) for culturing MG63 cells was prepared by immersing a titanium alloy plate with the plasma sprayed Ca(2)SiO(4) coatings in DMEM solution. The effect of the ionic products on cellular differentiation, collagen production, and local growth factors (prostaglandin E(2) [PGE(2)] and transforming growth factor-β [TGF-β1]) of osteoblast-like MG63 cells were investigated. The normal DMEM was also used to culture MG63 cells as the control group. Differentiation of cell was evaluated by detecting alkaline phosphatase (ALP) activity and osteocalcin (OC) synthesis as well as their gene expression. Collagen production was analyzed by Sircol assay. The levels of PGE(2) and TGF-β1 in culture medium were measured using enzyme-linked immunosorbent assay (ELISA). The gene expressions of TGF-β receptors (TGF-β RI and TGF-β RII) were also measured by real-time PCR technology. MG63 cells cultured in DMEM containing ionic products of Ca(2)SiO(4) coating showed enhanced differentiation and increased collagen production. The results obtained from ELISA showed that the levels of PGE(2) and TGF-β1 in experimental group were higher than that in control. The gene expression of TGF-β receptors was upregulated, indicating that more TGF-β1 bonded to their receptors which produce more effects on the osteoblastic activity, leading to enhanced differentiation and synthetic activity of osteoblast. It is concluded that ionic products of Ca(2)SiO(4) coating may enhance cellular differentiation and collagen production by influencing TGF-β1 pathway.

Bone regeneration and stem cells

This invited review covers research areas of central importance for orthopaedic and maxillofacial bone tissue repair, including normal fracture healing and healing problems, biomaterial scaffolds for tissue engineering, mesenchymal and foetal stem cells, effects of sex steroids on mesenchymal stem cells, use of platelet-rich plasma for tissue repair, osteogenesis and its molecular markers. A variety of cells in addition to stem cells, as well as advances in materials science to meet specific requirements for bone and soft tissue regeneration by addition of bioactive molecules, are discussed.

Platelet-rich plasma impairs osteoclast generation from human precursors of peripheral blood

Platelet-rich plasma is used to accelerate bone repair for the release of osteogenic growth factors from activated platelets. To date, the effects on osteoclasts have been only scarcely investigated, even though these cells are crucial for bone remodeling. The aim of this research was the evaluation of the effects of thrombin-activated platelets (PRP) on osteoclastogenesis from human blood precursors. We evaluated both the ability to influence osteoclast differentiation induced by the receptor activator of nuclear factor-kappaB ligand (RANKL), and the ability to induce osteoclast differentiation without RANKL. In both assays, the incubation with PRP supernatant at 10% did not significantly affect the formation of tartrate-resistant acid phosphatase (TRACP)-positive multinucleated cells that were able to form the F-actin ring. However, when PRP at 25 and 50% was added to the medium without RANKL, the generation of TRACP-positive multinucleated cells was inhibited. PRP, even at 10%, reduced the osteoclast-mediated bone collagen degradation, suggesting inhibition of osteoclast activation. Similarly, after incubation with PRP supernatant, calcitonin receptor mRNA was lower than the untreated samples. In conclusion, PRP at 10% interfered with the complete differentiation process of human osteoclast precursors. At higher concentration it impaired osteoclast formation also at an early stage of differentiation.

Cytokine expression pattern in compression and tension sides of the periodontal ligament during orthodontic tooth movement in humans

Orthodontic tooth movement is achieved by the remodeling of periodontal ligament (PDL) and alveolar bone in response to mechanical loading and is believed to be mediated by several host mediators, such as cytokines. By means of real-time polymerase chain reaction (PCR), we studied the pattern of expression of mRNA encoding several pro- and anti-inflammatory cytokines in relation to several extracellular matrix and bone remodeling markers, in tension (T) and compression (C) sides of the PDL of human teeth subjected to rapid maxillary expansion. The PDL of normal teeth was used as a control. The results showed that both T and C sides exhibited significantly higher expression of all targets when compared with controls, except for type I collagen (COL-I) and tissue inhibitor of metalloproteinase-1 (TIMP-1) on the C side. Comparing C and T sides, the C side exhibited higher expression of tumor necrosis factor-alpha (TNF-alpha), receptor activator of nuclear factor-kappaB ligand (RANKL), and matrix metalloproteinase-1 (MMP-1), whereas the T side presented higher expression of interleukin-10 (IL-10), TIMP-1, COL-I, osteoprotegerin (OPG), and osteocalcin (OCN). The expression of transforming growth factor-beta (TGF-beta) was similar in both C and T sides. Our data demonstrate a differential expression of pro- and anti-inflammatory cytokines in compressed and stretched PDL during orthodontic tooth movement.

Cancer associated fibroblasts stimulated by transforming growth factor beta1 (TGF-beta 1) increase invasion rate of tumor cells: a population study

Cancer associated fibroblasts (CAFs) are believed to promote tumor growth and progression. Our objective was to measure the effect of TGF-beta1 on fibroblasts isolated from invasive breast cancer patients. Fibroblasts were isolated from tissue obtained at surgery from patients with invasive breast cancer (CAF; n = 28) or normal reduction mammoplasty patients (normal; n = 10). Myofibroblast activation was measured by counting cells immunostained for smooth muscle alpha actin (ACTA2) in cultures +/- TGF-beta 1. Conditioned media (CM) was collected for invasion assays and RNA was isolated from cultures incubated in media +/- TGF-beta1 for 24 h. Q-PCR was used to measure expression of cyclin D1, fibronectin, laminin, collagen I, urokinase, stromelysin-1, and ACTA2 genes. Invasion rate was measured in chambers plated with MDA-MB-231 cells and exposed to CM in the bottom chamber; the number of cells that invaded into the bottom chamber was counted. Wilcox Rank Sum tests were used to evaluate differences in CAFs and normal fibroblasts and the effect of TGF-beta 1. There was no difference in percent myofibroblasts or invasion rate between normal and CAF cultures. However, TGF-beta1 significantly increased the percent of myofibroblasts (P < 0.01) and invasion rate (P = 0.02) in CAF cultures. Stromelysin-1 expression was significantly higher in normal versus CAF cultures (P < 0.01). TGF-beta 1 significantly increased ACTA2 expression in both normal and CAF cultures (P < 0.01). Expression of fibronectin and laminin was significantly increased by TGF-beta in CAF cultures (P < 0.01). CAFs were measurably different from normal fibroblasts in response to TGF-beta 1, suggesting that TGF-beta stimulates changes in CAFs that foster tumor invasion.

Prostaglandin E2 enhances transforming growth factor-beta 1 and TGF-beta receptors synthesis: an in vivo and in vitro study

The aims of this study were to determine how Prostaglandin E2 (PGE2) locally applied affected the immunodistribution of latent transforming growth factor-beta 1 (TGF-beta1), and how the eicosanoid modified TGF-beta1 release and TGF-beta receptors gene expression in cultured osteoblasts. PGE2 locally delivered on the rat mandible at doses of 0.1 and 0.05 mg/day, but not 0.025 mg/day, over 20 days significantly increased latent TGF-beta1 immunodistribution (P<0.001), comparing with a placebo-treated group. Cultured osteoblasts stimulated with 10(-5) or 10(-7)M PGE2 significantly varied the level of activated TGF-beta1 released into supernatants at different experimental periods compared with negative and positive controls. TGF-beta receptor type I gene expression was significantly increased in osteoblasts (P<0.01) after 10 days of treatment with 10(-5) and 10(-7)M PGE2, whereas 10(-3) M PGE2 produced the opposite effect. It is concluded that PGE2 may stimulate bone deposition by affecting TGF-beta pathway. This effect on the pathway appears to be dose-dependent.

Transforming growth factor-beta1 to the bone

TGF-beta1 is a ubiquitous growth factor that is implicated in the control of proliferation, migration, differentiation, and survival of many different cell types. It influences such diverse processes as embryogenesis, angiogenesis, inflammation, and wound healing. In skeletal tissue, TGF-beta1 plays a major role in development and maintenance, affecting both cartilage and bone metabolism, the latter being the subject of this review. Because it affects both cells of the osteoblast and osteoclast lineage, TGF-beta1 is one of the most important factors in the bone environment, helping to retain the balance between the dynamic processes of bone resorption and bone formation. Many seemingly contradictory reports have been published on the exact functioning of TGF-beta1 in the bone milieu. This review provides an overall picture of the bone-specific actions of TGF-beta1 and reconciles experimental discrepancies that have been reported for this multifunctional cytokine.

In Vitro, Ex Vivo, andIn VivoMethodological Approaches for Studying Therapeutic Targets of Osteoporosis and Degenerative Joint Diseases: How Biomarkers Can Assist?

Although our approach to the clinical management of osteoporosis (OP) and degenerative joint diseases (DJD)-major causes of disability and morbidity in the elderly-has greatly advanced in the past decades, curative treatments that could bring ultimate solutions have yet to be found or developed. Effective and timely development of candidate drugs is a critical function of the availability of sensitive and accurate methodological arsenal enabling the recognition and quantification of pharmacodynamic effects. The established concept that both OP and DJD arise from an imbalance in processes of tissue formation and degradation draws attention to need of establishing in vitro, ex vivo, and in vivo experimental settings, which allow obtaining insights into the mechanisms driving increased bone and cartilage degradation at cellular, organ, and organism levels. When addressing changes in bone or cartilage turnover at the organ or organism level, monitoring tools adequately reflecting the outcome of tissue homeostasis become particularly critical. In this context, bioassays targeting the quantification of various degradation and formation products of bone and cartilage matrix elements represent a useful approach. In this review, a comprehensive overview of widely used and recently established in vitro, ex vivo, and in vivo set-ups is provided, which in many cases effectively take advantage of the potentials of biomarkers. In addition to describing and discussing the advantages and limitations of each assay and their methods of evaluation, we added experimental and clinical data illustrating the utility of biomarkers for these methodological approaches.

Chronology and regulation of gene expression of RANKL in the rat dental follicle

Tooth eruption in the rat requires bone resorption resulting from a major burst of osteoclastogenesis on postnatal day 3 and a minor burst of osteoclastogenesis on postnatal day 10 in the alveolar bone of the first mandibular molar. The dental follicle regulates the major burst on postnatal day 3 by down-regulating its osteoprotegerin (OPG) gene expression to enable osteoclastogenesis to occur. To determine the role of receptor activator of nuclear factor-kappa B ligand (RANKL) in tooth eruption, its gene expression was measured on postnatal days 1-11 in the dental follicle. The results show that RANKL expression was significantly elevated on postnatal days 9-11 in comparison to low expression levels at earlier time-points. As OPG expression is high at this latter time-point, this increase in RANKL expression would be needed for stimulating the minor burst of osteoclastogenesis. Tumor necrosis factor-alpha enhances RANKL gene expression in vitro and it may be responsible for up-regulating RANKL in vivo. Transforming growth factor-beta1 and interleukin-1alpha also enhance RANKL gene expression in vitro but probably have no effect in vivo because they are maximally expressed early. Bone morphogenetic protein-2 acts to down-regulate RANKL expression in vitro and, in vivo, may promote alveolar bone growth in the basal region of the tooth.

Plasma levels of platelet-derived growth factor BB and transforming growth factor in patients with failed hip prostheses

BACKGROUND

The role of growth factors in prosthesis loosening is unclear. We evaluated the levels of platelet-derived growth factor BB (PDGF-BB), transforming growth factors beta1 (TGF-beta1) and beta2 (TGF-beta2), both before and after activation, in patients with aseptic loosening of their hip prosthesis.

PATIENTS AND METHODS

26 patients with loosened hip implants were compared with 21 patients who had stable hip prostheses, and 28 patients undergoing primary hip replacement. The plasma levels of the growth factors were analyzed by enzyme immunoassay. TGF-beta1 and TGF-beta2 were determined both before and after activation.

RESULTS

Patients with aseptic loosening had significantly lower PDGF-BB levels than patients undergoing primary hip replacement, and significantly lower TGF-beta2 levels than patients with a stable implant. Patients with stable prostheses had significantly higher TGF-beta1 and TGF-beta2 levels than patients undergoing primary hip replacement.

INTERPRETATION

It is possible that the prosthetic implant itself causes a local increase in PDGF-BB, TGF-beta1 and TGF-beta2, released by osteoblasts and other cells in the microenvironment. The plasma PDGF-BB measured does not correspond to local release, which is probably due to local consumption or degradation. The consumption of PDGF-BB is low in stable implants, and TGF-beta1 and TGF-beta2 levels increase during bone formation. In loosening, PDGF-BB consumption is higher and causes a significant reduction in plasma levels as compared to presurgery. The formation of poor-quality bone may be related to the scarce increase in TGF-beta1 and TGF-beta2. In conclusion, compared with patients with a stable implant, a reduction in bone-forming growth factors appears to occur in individuals with aseptic loosening.

Extracellular nucleotides act through P2X7 receptors to activate NF-kappaB in osteoclasts

Nucleotides, released in response to mechanical and other stimuli, act on P2 receptors in osteoclasts and other cell types. In vitro studies of osteoclasts from rabbits and P2X7 receptor-deficient mice revealed that P2X7 receptors couple to activation of the key transcription factor NF-kappaB.

INTRODUCTION

Osteoclasts express functional P2X4 and P2X7 receptors, which are ATP-gated cation channels. Knockout (KO) of the P2X7 receptor has revealed its role in regulating bone formation and resorption, but the underlying signals are not known. The transcription factor NF-kappaB plays a key role in the response of osteoclasts to RANKL and other cytokines. The aim of this study was to examine whether P2X receptors on osteoclasts signal through NF-kappaB.

MATERIALS AND METHODS

Osteoclasts were isolated from neonatal rabbits or wildtype (WT) and P2X7 receptor KO mice. Immunofluorescence was used to detect the p65 subunit of NF-kappaB, which, on activation, translocates from the cytosol to the nuclei. The concentration of cytosolic free Ca2+ ([Ca2+]i) was monitored in single osteoclasts loaded with fura-2.

RESULTS

In control samples, few rabbit osteoclasts demonstrated nuclear localization of NF-kappaB. Benzoyl-benzoyl-ATP (BzATP, a P2X7 agonist, 300 microM) induced nuclear translocation of NF-kappaB after 3 h in approximately 45% of rabbit osteoclasts. In contrast, a low concentration of ATP (10 microM, sufficient to activate P2X4 and P2Y2, but not P2X7 receptors) did not induce nuclear translocation of NF-kappaB. Because BzATP activates multiple P2 receptors, we examined responses of osteoclasts derived from WT and P2X7 receptor KO mice. Treatment with BzATP for 30 minutes increased nuclear localization of NF-kappaB in osteoclasts from WT but not KO mice, showing involvement of P2X7 receptors. Both ATP (10 microM) and BzATP (300 microM) caused transient elevation of [Ca2+]i, indicating that rise of calcium alone is not sufficient to activate NF-kappaB. Pretreatment of rabbit osteoclasts with osteoprotegerin inhibited translocation of NF-kappaB induced by RANKL but not by BzATP, establishing that the effects of BzATP are independent of RANKL signaling.

CONCLUSION

These findings show that P2X7 nucleotide receptors couple to activation of NF-kappaB in osteoclasts. Thus, nucleotides, released at sites of inflammation or in response to mechanical stimuli, may act through NF-kappaB to regulate osteoclast formation and activity.

Vacuolar H+-ATPase binding to microfilaments: regulation in response to phosphatidylinositol 3-kinase activity and detailed characterization of the actin-binding site in subunit B

Vacuolar H(+)-ATPase (V-ATPase) binds microfilaments, and that interaction may be mediated by an actin binding domain in subunit B of the enzyme. To test for possible physiologic functions of the actin binding activity of V-ATPase, early responses of resorbing osteoclasts to inhibition of phosphatidylinositol 3-kinase activity by wortmannin and LY294002 were examined. Rapid co-localization between V-ATPase and F-actin was demonstrated by immunocytochemistry, and corresponding association between V-ATPase and F-actin in immunoprecipitations and pelleting assays was detected. This response was reversed as osteoclasts recovered resorptive activity after inhibitors were removed. By expressing and characterizing fusion proteins containing segments of the actin-binding amino-terminal regions of the B subunits of V-ATPase, we mapped the actin-binding site to a 44-amino acid domain. An 11-amino acid segment with a sequence similar to the actin-binding site of human profilin I was detected within this region. 13-Mers containing these profilin-like segments bound actin in fluorescent anisotropy studies and competed with profilin for binding to actin. Using site-directed mutagenesis, the 11-amino acid profilin-like actin-binding motifs (amino acids 49-59 of B1 and 55-65 of B2) were replaced with an 11-amino acid spacer with a sequence based on the homologous sequence from subunit B of Pyrococcus horikoshii, an organism that lacks an actin cytoskeleton. These substitutions eliminated the actin-binding activity of the B subunit fusion proteins. In summary, binding between V-ATPase and F-actin in osteoclasts occurs in response to blocking phosphatidylinositol 3-kinase activity. This response was fully reversible. The actin binding activities of the B subunits of V-ATPase required 11-amino acid actin-binding motifs that are similar in sequence to the actin-binding site of mammalian profilin I.

RANKL and Vascular Endothelial Growth Factor (VEGF) Induce Osteoclast Chemotaxis through an ERK1/2-dependent Mechanism

Development of bone depends on a continuous supply of bone-degrading osteoclasts. Although several factors such as the matrix metalloproteinases and the integrins have been shown to be important for osteoclast recruitment, the mechanism of action remains poorly understood. In this study we investigated the molecular mechanisms homing osteoclasts to their future site of resorption during bone development. We show that RANKL and VEGF, two cytokines known to be present in bone, possess chemotactic properties toward osteoclasts cultured in modified Boyden chambers. Furthermore, in ex vivo cultures of embryonic murine metatarsals, a well established model of osteoclast recruitment, antagonists of RANKL and VEGF reduced calcium release, showing that both cytokines play roles during bone development. In cultures of purified osteoclasts both RANKL and VEGF induced phosphorylation of ERK1/2 MAP kinase. M-CSF, a well-known chemoattractant of osteoclast, also induced activation of ERK1/2, although this activation followed a kinetic pattern differing from that of RANKL and VEGF. RANKL and VEGF-induced, but not M-CSF-induced, osteoclast invasion was completely blocked by the specific inhibitor of ERK1/2 phosphorylation, PD98059. In addition, PD98059 was able to inhibit calcium release in cultures of embryonic metatarsals. In contrast, PD98059 was unable to abrogate the RANKL-induced calcium release in the tibia model, demonstrating that only some of the RANKL functions on osteoclast physiology are regulated through the ERK1/2 pathway. Taken together, these results show that RANKL and VEGF, in addition to their role in osteoclast differentiation and activation of resorption, are important components of the processes regulating osteoclast chemotaxis.

Transforming growth factor-beta controls human osteoclastogenesis through the p38 MAPK and regulation of RANK expression

Although RANK-L is essential for osteoclast formation, factors such as transforming growth factor-beta (TGF-beta) are potent modulators of osteoclastogenic stimuli. To systematically investigate the role of TGF-beta in human osteoclastogenesis, monocytes were isolated from peripheral blood by three distinct approaches, resulting in either a lymphocyte-rich, a lymphocyte-poor, or a pure osteoclast precursor (CD14-positive) cell population. In each of these osteoclast precursor populations, the effect of TGF-beta on proliferation, TRAP activity, and bone resorption was investigated with respect to time and length of exposure. When using the highly pure CD14 osteoclast precursor cell population, the effect of TGF-beta was strongly dependent on the stage of osteoclast maturation. When monocytes were exposed to TGF-beta during the initial culture period (days 1-7), TRAP activity and bone resorption were increased by 40%, whereas the cell number was reduced by 25%. A similar decrease in cell number was observed when TGF-beta was present during the entire culture period (days 1-21), but in direct contrast, TRAP activity, cell fusion, cathepsin K, and matrix metalloproteinase (MMP)-9 expression as well as bone resorption were almost completely abrogated. Moreover, we found that latent TGF-beta was strongly activated by incubation with MMP-9 and suggest this to be a highly relevant mechanism for regulating osteoclast activity. To further investigate the molecular mechanism responsible for the divergent effects of continuous versus discontinuous exposure to TGF-beta, we examined RANK expression and p38 MAPK activation. We found the TGF-beta strongly induced p38 MAPK in monocytes, but not in mature osteoclasts, and that continuous exposure of TGF-beta to monocytes down-regulated RANK expression. The current results suggest that TGF-beta promotes human osteoclastogenesis in monocytes through stimulation of the p38 MAPK, whereas continuous exposure to TGF-beta abrogates osteoclastogenesis through down-regulation of RANK expression and therefore attenuation of RANK-RANK-L signaling.

Matrix metalloproteinases (MMP) and cathepsin K contribute differently to osteoclastic activities

The best established proteolytic event of osteoclasts is bone matrix solubilization by the cysteine proteinase cathepsin K. Here, however, we draw the attention on osteoclastic activities depending on matrix metalloproteinases (MMPs). We discuss the observations supporting that MMPs contribute significantly to bone matrix solubilization in specific areas of the skeleton and in some developmental and pathological situations. Our discussion takes into account (1) the characteristics of the bone remodeling persisting in the absence of cathepsin K, (2) the ultrastructure of the resorption zone in response to inactivation of MMPs and of cathepsin K in different bone types, (3) bone resorption levels in MMP knockout mice compared to wild-type mice, (4) the identification of MMPs in osteoclasts and surrounding cells, and (5) the effect of different bone pathologies on the serum concentrations of specific collagen fragments believed to discriminate between cathepsin K and MMP cleavage. Next, we provide evidence that MMPs are very critical for osteoclast migration, thereby controlling also the cell-matrix interactions required for cell attachment/detachment. The evidence supporting this role is based on a model of osteoclast recruitment in primitive long bones, an assay of osteoclast invasion through collagen gel, and the effect of proteinase inhibitors/knockouts in these models. Furthermore, we mention observations indicating a role of MMPs in initiation of bone resorption. Finally, we emphasize the many distinct ways MMPs may alter focally the extracellular environment thereby regulating the osteoclast behavior. Although the understanding of MMPs in osteoclast biology is rapidly expanding, it is suspected that important roles remain to be discovered.

Higher stromal expression of transforming growth factor-beta type II receptors is associated with poorer prognosis breast tumors

Transforming growth factor-beta (TGFB) is a potent inhibitor of normal epithelial cell proliferation, and may be one of the regulatory factors that are perturbed during tumor development. While many tumor cell lines no longer respond to the inhibitory effects of TGFB due to a reduction or absence of the type II receptor (TGFBR2), the role of TGFBR2 in tumors from patients with breast cancer is less clear. The objective of this study was to screen human breast tumors to determine if there was a TGFBR2 mutation and/or altered expression of TGFBR2 protein. Using 10 unique primers, SSCP-PCR was used to detect heterozygosity in the complete coding sequence from 72 tumors and normal DNA from 20 individuals. One region of the promoter was also examined. Expression of TGFBR2 in the same breast tumors was examined by immunohistochemistry. Sequence variations were identified among normal and tumor tissue samples by SSCP-PCR within coding regions of exon 4 (1/72 samples) and within non-coding regions of intron 2 (1/72), intron 3 (72/72), and intron 6 (1/72). A new polymorphism was identified in intron 3. Observed allele frequencies were consistent with Hardy-Weinberg equilibrium in both the tumors and normal DNA. TGFBR2 was expressed in the epithelium and stroma of tumor tissue. The percentage of cells expressing TGFBR2 in stroma was higher in patients that had a positive lymph node status and/or negative estrogen and progesterone receptor expression. There was no relationship between TGFBR2 expression in the epithelium and these variables.

Participation of protein kinase C beta in osteoclast differentiation and function

Protein kinase C (PKC) proteins have been shown to be involved in diverse cellular responses of various cell types. In experiments to identify genes regulated during osteoclast differentiation by a cDNA microarray approach, we found that the gene expression of PKC-betaII was upregulated in differentiated cells. Reverse transcription-polymerase chain reaction and Western blotting analyses also showed an increase in PKC-betaI as well as PKC-betaII during osteoclast formation in mouse bone marrow cell cultures in the presence of macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor-kappaB ligand (RANKL). Use of an antisense oligonucleotide to PKC-betaII resulted in a reduction in the RANKL-driven osteoclastogenesis. Pharmacological intervention with PKC-beta activity by the specific inhibitor CG53353 suppressed cellular differentiation and fusion processes during osteoclastogenesis and inhibited bone-resorbing function of mature osteoclasts. PKC-beta inhibition abolished the ERK and MEK activation by macrophage-colony stimulating factor and RANKL in osteoclast precursor cells whereas the cytokine-induced NF-kappaB activation was not hampered by the PKC-beta inhibition. Our findings indicate that PKC-beta has a role in regulation of osteoclast formation and function potentially by participating in the ERK signaling pathway of M-CSF and RANKL.

Nerve-mast cell (RBL) interaction: RBL membrane ruffling occurs at the contact site with an activated neurite

Mast cell-neurite interaction serves as a model for neuroimmune interaction. We have shown that neurite-mast cell communication can occur via substance P interacting with neurokinin (NK)-1 receptors on the mucosal mast cell-like cell, the rat basophilic leukemia (RBL) cell. Neurite (murine superior cervical ganglia) and RBL cell [expressing the granule-associated antigen CD63-green fluorescent protein (GFP) conjugate] cocultures were established and stimulated with bradykinin (BK; 10 nM) or scorpion venom (SV; 10 pg/ml), both of which activate only neurites. Cell activation was assessed by confocal imaging of Ca2+ (cells preloaded with fluo 3), and analyses of RBL CD63-GFP+ granule movement were conducted. Neurite activation by BK or SV was followed by RBL Ca2+ mobilization, which was inhibited by an NK-1 receptor antagonist (NK-1 RA). Moreover, membrane ruffling was observed on RBL pseudopodial extensions in contact with the activated neurite, but not on noncontacting pseudopodia. RBL membrane ruffling was inhibited by NK-1 RA, but not NK-2 RA, and was accompanied by a significant increase in granule movement (0.13 +/- 0.04 vs. 0.05 +/- 0.01 microm/s) that was most evident at the point of neurite contact: many of the granules moved toward the plasmalemma. This is the first documentation of such precise (restricted to the membrane's contact site) transfer of information between nerves and mast cells that could allow for very subtle in vivo communication between these two cell types.