Inhibiting and stimulating effects of TGF-beta 1 on osteoclastic bone resorption in fetal mouse bone organ cultures

Potential Targets and Mechanisms of Jiedu Quyu Ziyin Decoction for Treating SLE-GIOP: Based on Network Pharmacology and Molecular Docking

Background

Systemic lupus erythematosus (SLE) is characterized by poor regulation of the immune response leading to chronic inflammation and multiple organ dysfunction. Glucocorticoid (GC) is currently one of the main treatments. However, a high dose or prolonged use of GC may result in glucocorticoid-induced osteoporosis (GIOP). Jiedu Quyu Ziyin decoction (JP) is effective in treating SLE and previous clinical studies have proved that JP can prevent and treat SLE steroid osteoporosis (SLE-GIOP). We aim to examine JPs main mechanism on SLE-GIOP through network pharmacology and molecular docking.

Methods

TCMSP and TCMID databases were used to screen potential active compounds and targets of JP. The SLE-GIOP targets are collected from GeneCards, OMIM, PharmGkb, TTD, and DrugBank databases. R software was used to obtain the cross-targets of JP and SLE-GIOP and to perform GO and KEGG enrichment analysis. Cytoscape software was used to make the Chinese Medicines-Active Ingredient-Intersection Targets network diagram. STRING database construct protein-protein interaction network and obtain the core targets. Auto Dock Tools and Pymol software were used for docking.

Results

Fifty eight targets overlapped between JP and SLE-GIOP were suggested as potential targets of JP in the treatment of SLE-GIOP. Network topology analysis identified five core targets. GO enrichment analysis was obtained 1,968 items, and the top 10 biological process, closeness centrality, and molecular function were displayed. A total of 154 signaling pathways were obtained by KEGG enrichment analysis, and the top 30 signaling pathways were displayed. JP was well bound by MAPK1, TP53, and MYC according to the molecular docking results.

Conclusion

We investigated the potential targets and signaling pathways of JP against SLE-GIOP in this study. It shows that JP is most likely to achieve the purpose of treating SLE-GIOP by promoting the proliferation and differentiation of osteoblasts. A solid theoretical foundation will be provided for the future study of clinical and experimental topics.

An insight into the implications of estrogen deficiency and transforming growth factor β in antiepileptic drugs-induced bone loss

There have been a number of reports that chronic antiepileptic drug (AEDs) therapy is associated with abnormal bone and calcium metabolism, osteoporosis/osteomalacia, and increased risk of fractures. Bony adverse effects of long term antiepileptic drug therapy have been reported for more than four decades but the exact molecular mechanism is still lacking. Several mechanisms have been proposed regarding AEDs induced bone loss; Hypovitaminosis D, hyperparathyroidism, estrogen deficiency, calcitonin deficiency. Transforming growth factor-β (TGF- β) is abundant in bone matrix and has been shown to regulate the activity of osteoblasts and osteoclasts in vitro. All isoforms of TGF- β are expressed in bone and intricately play role in bone homeostasis by modulating estrogen level. Ovariectomised animal have shown down regulation of TGF- β in bone that could also be a probable target of AEDs therapy associated bone loss. One of the widely accepted hypotheses regarding the conventional drugs induced bone loss is hypovitaminosis D which is by virtue of their microsomal enzyme inducing effect. However, despite of the lack of enzyme inducing effect of certain newer antiepileptic drugs, reduced bone mineral density with these drugs have also been reported. Thus an understanding of bone biology, pathophysiology of AEDs induced bone loss at molecular level can aid in the better management of bone loss in patients on chronic AEDs therapy. This review focuses mainly on certain new molecular targets of AEDs induced bone loss.

Decompression effects on bone healing in rat mandible osteomyelitis

Osteomyelitis (OM) of the jaw is usually caused by a chronic odontogenic infection. Decompression is the release the intraluminal pressure in the cystic cavity allowing gradual bone growth from the periphery. The aim of this study was to analyze the effectiveness of decompression in an OM jaw model. A 4-mm-diameter defect was made on mandibles of fourteen Sprague-Dawley rats and inoculated with S. aureus (20 μl of 1 × 10⁷ CFU/ml) injection. Two weeks later, four groups were made as non-treatment (C1), only curettage (C2), curettage and decompression (E1), and curettage and decompression with normal saline irrigation (E2). After four weeks, each group was analyzed. Most micro-CT parameters, including bone mineral density [0.87 (± 0.08) g/cm³] with bone volume [0.73 (± 0.08) mm³] was higher in E2 group than that of C1 group (p = 0.04, p = 0.05, respectively). E2 group in histology showed the highest number of osteocytes than those of control groups, 91.00 (± 9.90) (p = 0.002). OPN were expressed strongly in the E1 ("5": 76-100%) that those of other groups. Decompression drains induced advanced bone healing compared to that of curettage alone. Therefore, it could be recommended to use decompressive drain for enhancing the jaw OM management.

The role of TGFβ receptor 1-smad3 signaling in regulating the osteoclastic mode affected by fluoride

Studies that have focused on the role TGFβ signaling plays in osteoclast activity are gradually increasing; however, literature is rare in terms of fluorosis. The aim of this study is to observe the role the TβR1/Smad3 pathway plays in fluoride regulating cellsosteoclast-like cells that are under the treatment of TGFβ receptor 1 kinase. The RANKL-mediated osteoclast-like cells from RAW264.7 cells were used as osteoclast precursor model. The profile of miRNA expression in fluoride-treated osteoclast-like cells exhibited 303 upregulated miRNAs, 61 downregulated miRNAs, and further drew 37 signaling pathway maps by KEGG and Biocarta pathway enrichment analysis. TGFβ and its downstream effectors were included among them. Osteoclast viability, formation and function were detected via MTT method, bone resorption pit and tartrate-resistant acid phosphatase (TRACP) staining, respectively. Results demonstrated that different doses of fluoride exhibited a biphasic effect on osteoclast cell viability, differentiation, formation and function. It indicated that a low dose of fluoride treatment stimulated them, but high dose inhibited them. SB431542 acted as TβR1 kinase inhibitor and blocked viability, formation and function of osteoclast-like cells regulated by fluoride. The expression of the osteoclast marker, RANK, and TβR1/Smad3 at gene and protein level was analyzed under fluoride with and without SB431542 treatment. Fluoride treatment indicated little effect on the RANK protein expression; however it significantly influenced TRACP expression in osteoclast-like cells. The stimulation of fluoride on the expression of Smad3 gene and phosphorylated Smad3 protein exhibited dose-dependent manner. SB431542 significantly impeded phosphorylation of Smad3 protein and TRACP expression in osteoclast-like cells that were exposed to fluoride. Our work demonstrated that TGFβ signaling played a key role in fluoride regulating osteoclast differentiation, formation and function. It elucidated that TβR1/Smad3 pathway participated in the mechanism of biphasic modulation of osteoclast mode regulated by fluoride.

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.

Osteoclasts-Key Players in Skeletal Health and Disease

The differentiation of osteoclasts (OCs) from early myeloid progenitors is a tightly regulated process that is modulated by a variety of mediators present in the bone microenvironment. Once generated, the function of mature OCs depends on cytoskeletal features controlled by an αvβ3-containing complex at the bone-apposed membrane and the secretion of protons and acid-protease cathepsin K. OCs also have important interactions with other cells in the bone microenvironment, including osteoblasts and immune cells. Dysregulation of OC differentiation and/or function can cause bone pathology. In fact, many components of OC differentiation and activation have been targeted therapeutically with great success. However, questions remain about the identity and plasticity of OC precursors and the interplay between essential networks that control OC fate. In this review, we summarize the key principles of OC biology and highlight recently uncovered mechanisms regulating OC development and function in homeostatic and disease states.

The osteoimmunology of alveolar bone loss

The mineralized structure of bone undergoes constant remodeling by the balanced actions of bone-producing osteoblasts and bone-resorbing osteoclasts (OCLs). Physiologic bone remodeling occurs in response to the body's need to respond to changes in electrolyte levels, or mechanical forces on bone. There are many pathological conditions, however, that cause an imbalance between bone production and resorption due to excessive OCL action that results in net bone loss. Situations involving chronic or acute inflammation are often associated with net bone loss, and research into understanding the mechanisms regulating this bone loss has led to the development of the field of osteoimmunology. It is now evident that the skeletal and immune systems are functionally linked and share common cells and signaling molecules. This review discusses the signaling system of immune cells and cytokines regulating aberrant OCL differentiation and activity. The role of these cells and cytokines in the bone loss occurring in periodontal disease (PD) (chronic inflammation) and orthodontic tooth movement (OTM) (acute inflammation) is then described. The review finishes with an exploration of the emerging role of Notch signaling in the development of the immune cells and OCLs that are involved in osteoimmunological bone loss and the research into Notch signaling in OTM and PD.

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.

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.

Osteoclast activity and subtypes as a function of physiology and pathology--implications for future treatments of osteoporosis

Osteoclasts have traditionally been associated exclusively with catabolic functions that are a prerequisite for bone resorption. However, emerging data suggest that osteoclasts also carry out functions that are important for optimal bone formation and bone quality. Moreover, recent findings indicate that osteoclasts have different subtypes depending on their location, genotype, and possibly in response to drug intervention. The aim of the current review is to describe the subtypes of osteoclasts in four different settings: 1) physiological, in relation to turnover of different bone types; 2) pathological, as exemplified by monogenomic disorders; 3) pathological, as identified by different disorders; and 4) in drug-induced situations. The profiles of these subtypes strongly suggest that these osteoclasts belong to a heterogeneous cell population, namely, a diverse macrophage-associated cell type with bone catabolic and anabolic functions that are dependent on both local and systemic parameters. Further insight into these osteoclast subtypes may be important for understanding cell-cell communication in the bone microenvironment, treatment effects, and ultimately bone quality.

TGF-β1 as possible link between loss of bone mineral density and chronic inflammation

Background

The TGF family plays a key role in bone homeostasis. Systemic or topic application of proteins of this family apparently positively affects bone healing in vivo. However, patients with chronic inflammation, having increased TGF-β₁ serum-levels, often show reduced bone mineral content and disturbed bone healing. Therefore, we wanted to identify intracellular mechanisms induced by chronic presence of TGF-β₁ and their possible role in bone homeostasis in primary human osteoblasts.

Methodology/Principal Findings

Osteoblasts were isolated from femur heads of patients undergoing total hip replacement. Adenoviral reporter assays showed that in primary human osteoblasts TGF-β₁ mediates its signal via Smad2/3 and not Smad1/5/8. It induces proliferation as an intermediate response but decreases AP-activity and inorganic matrix production as a late response. In addition, expression levels of osteoblastic markers were strongly regulated (AP↓; Osteocalcin↓; Osteopontin↑; MGP↓; BMP 2↓; BSP2↓; OSF2↓; Osteoprotegerin↓; RANKL↑) towards an osteoclast recruiting phenotype. All effects were blocked by inhibition of Smad2/3 signaling with the Alk5-Inhibitor (SB431542). Interestingly, a rescue experiment showed that reduced AP-activities did not recover to base line levels, even 8 days after stopping the TGF-β₁ application.

Conclusions/Significance

In spite of the initial positive effects on cell proliferation, it is questionable if continuous Smad2/3 phosphorylation is beneficial for bone healing, because decreased AP-activity and BMP2 levels indicate a loss of function of the osteoblasts. Thus, inhibition of Smad2/3 phosphorylation might positively influence functional activity of osteoblasts in patients with chronically elevated TGF-β₁ levels and thus, could lead to an improved bone healing in vivo.

Biochemical markers in preclinical models of osteoporosis

Although several treatments for osteoporosis exist, further understanding of the mode of action of current treatments, as well as development of novel treatments, are of interest. Thus, preclinical models of osteoporosis are very useful, as they provide the possibility for gaining knowledge about the cellular mechanisms underlying the disease and for studying pharmaceutical prevention or intervention of the disease in simple and strictly controlled systems. In this review, we present a comprehensive collection of studies using biochemical markers of bone turnover for investigation of preclinical models of osteoporosis. These range from pure and simple in vitro systems, such as osteoclast cultures, to ex vivo models, such as cultures of embryonic murine tibiae and, finally, to in vivo models, such as ovariectomy and orchidectomy of rats. We discuss the relevance of the markers in the individual models, and compare their responses to those observed using 'golden standard' methods.

Low transforming growth factor-beta1 serum levels in idiopathic male osteoporosis

INTRODUCTION

Although the etiology of osteoporosis is different between men and women, the underlying pathophysiological mechanism is similar, namely an absolute or relative increase in bone resorption, leading to progressive bone loss. Transforming growth factor (TGF)-beta1 is a growth factor in human bone, which is produced by osteoblasts, and which has various effects on osteoclasts and osteoblasts. The aim of our study was to determine serum TGF-beta1 levels in male patients with idiopathic osteoporosis.

METHODS

Twenty five males with idiopathic osteoporosis and 25 age-matched controls were studied. Osteoporosis was defined by a T score of <-2.5 in the lumbar spine or at the femoral neck. We measured levels of TGF-beta1, estradiol, total and bioactive testosterone. Various markers of bone remodeling were also measured.

RESULTS

TGF-beta1 was significantly lower in osteoporotic patients than in controls (3.706 ng/dl, 25-75 percentiles: 2.81-5.33 vs 8.659 ng/dl, 25-75 percentiles: 4.837-11.835; p=0.000). Moreover, TGF-beta1 levels were positively correlated with bone mineral density (BMD) at the femoral neck (r=0.439, p=0.028), and at the lumbar spine (r=0.41, p=0.042). No correlation was found between serum estradiol, testosterone and TGF-beta1 levels.

DISCUSSION

Serum TGF-beta1 levels are depressed in osteoporotic men and are positively correlated with hip and spine BMD. The results of our study suggest that TGF-beta1 may play a role in the pathogenesis of idiopathic male osteoporosis.

Anabolic agents and the bone morphogenetic protein pathway

A major unmet need in the medical field today is the availability of suitable treatments for the ever-increasing incidence of osteoporosis and the treatment of bone deficit conditions. Although therapies exist which prevent bone loss, the options are extremely limited for patients once a substantial loss of skeletal bone mass has occurred. Patients who have reduced bone mass are predisposed to fractures and further morbidity. The FDA recently approved PTH (1-34) (Teriparatide) for the treatment of postmenopausal osteoporosis after both preclinical animal and clinical human studies indicated it induces bone formation. This is the only approved bone anabolic agent available but unfortunately it has limited use, it is relatively expensive and difficult to administer. Consequently, the discovery of low cost orally available bone anabolic agents is critical for the future treatment of bone loss conditions. The intricate process of bone formation is co-ordinated by the action of many different bone growth factors, some stored in bone matrix and others released into the bone microenvironment from surrounding cells. Although all these factors play important roles, the bone morphogenetic proteins (BMPs) clearly play a central role in both bone cartilage formation and repair. Recent research into the regulation of the BMP pathway has led to the discovery of a number of small molecular weight compounds as candidate bone anabolic agents. These agents may usher in a new wave of more innovative and versatile treatments for osteoporosis as well as orthopedic and dental indications.

Assessment of cell proliferation during mandibular distraction osteogenesis in the maturing rat

INTRODUCTION

The cellular mechanisms controlling distraction osteogenesis are not well understood. The purpose of this study was to examine the role of cell proliferation in the regulation of mandibular distraction osteogenesis.

METHODS

Unilateral mandibular ramus osteotomies were performed on 125 3-month-old Sprague-Dawley rats. The rats were randomized into 4 distraction rate groups and distracted for 5 days after 3-day latency. Rats (7 or 8 from each rate group) were killed at 4 time points. The rats received 5-bromo-2-deoxyuridine (BrdU) injections (40 mg per kilogram, i.p.) at day 3 (end of latency).

RESULTS

Both intramembranous and endochondral ossification was seen in the osteogenesis area. BrdU+ mesenchymal progenitor cells were significantly higher at day 10 (P <.05) and were found most numerously around the sagittal middle portion of the gap (P <.01). The greatest numbers of BrdU+ osteocytes were seen at day 38 (P <.05). Both BrdU+ osteoclasts and chondrocytes peaked at day 24.

CONCLUSIONS

Mesenchymal progenitor cells are mostly recruited in the early consolidation period, but they decrease in the middle and late consolidation periods during mandibular distraction osteogenesis. The rapid rate might suppress or sustain the proliferation and differentiation of mesenchymal progenitor cells during mandibular distraction osteogenesis. BrdU+ cells can survive throughout the entire experimental period of 5 weeks.

Current insights into the role of transforming growth factor-beta in bone resorption

Transforming growth factor-beta (TGF-beta) elicits a variety of effects on cellular proliferation and differentiation. The major repository for TGF-beta is bone, where it possesses separate facilitative and suppressive actions on osteoclast differentiation and bone resorption. Without a direct enabling stimulus from TGF-beta monocytes cannot form osteoclasts but instead follow macrophage differentiation pathways. This facilitative action depends on an ability to promote a state in which precursors are resistant to anti-osteoclastic inflammatory signals. Following the initiation of resorption TGF-beta is released from bone matrix. This acts on osteoblasts to reduce the availability of the osteoclast differentiation factor, RANKL (receptor activator of NFkappaB ligand) and thereby indirectly limits further osteoclast formation. Thus TGF-beta has a fundamental role in the control of bone resorption having actions that first allow monocytes to develop into osteoclasts then subsequently limiting the extent and duration of resorption after its release from the bone matrix.

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.

Effects of 17beta-estradiol, tamoxifen and raloxifene on the protein and mRNA expression of interleukin-6, transforming growth factor-beta1 and insulin-like growth factor-1 in primary human osteoblast cultures

We investigated the effects of 17betaestradiol and two selective estrogen receptor modulators, tamoxifen and raloxifene, on the expression and release of constitutive and interleukin-1-stimulated interleukin (IL)-6, transforming growth factor-beta1 (TGF-beta1) and insulin-like growth factor-1 by osteoblasts in primary culture from trabecular bone of healthy post-menopausal women. After 24 h incubation with 10(-8) M concentration of these compounds, there was no decrease in: a) the constitutive or IL-1beta-induced levels of IL-6 protein released to culture medium; b) the constitutive IL-6 mRNA expression after incubation of osteoblasts with 10(-8) M 17betaestradiol or 10(-8) M tamoxifen for 1, 3, 6, 24 or 30 h. Although a decrease after 30 h of treatment with 10(-8) M, raloxifene was found in mRNA IL-6 expression, and this fact was not reflected by a decrease in the release of IL-6 protein to the culture medium after 48 h of incubation with 10(-8) M or 10(-7) M raloxifene. Tumoral growth factorTGF-betal expression was not influenced by incubation with these compounds. Gene expression of IGF-I increased following 24 or 30 h incubation with 10(-8) M 17beta-estradiol and 30 h incubation with raloxifene. Tamoxifen did not affect IGF-I expression. In conclusion, the effects of estradiol or tamoxifen on bone metabolism do not appear to be mediated through the regulation of osteoblast IL-6 release or synthesis, but raloxifene produces a decrease in mRNA IL-6 expression. The actions of estradiol, tamoxifen and raloxifene do not appear to be mediated by tumoral growth factor TGF-beta1. On the other hand, an increase in IGF-I synthesis induced by raloxifene and estradiol could mediate, in part, the effects of these compounds on bone.

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.

Osteocytes inhibit osteoclastic bone resorption through transforming growth factor-beta: enhancement by estrogen

Osteocytes are the most abundant cells in bone and distributed throughout the bone matrix. They are connected to the each other and to the cells on the bone surface. Thus, they may also secrete some regulatory factors controlling bone remodeling. Using a newly established osteocyte-like cell line MLO-Y4, we have studied the interactions between osteocytes and osteoclasts. We collected the conditioned medium (CM) from MLO-Y4 cells, and added it into the rat osteoclast cultures. The conditioned medium had no effect on osteoclast number in 24-h cultures, but it dramatically inhibited resorption. With 5, 10, and 20% CM, there was 25, 39, and 42% inhibition of resorption, respectively. Interestingly, the inhibitory effect was even more pronounced, when MLO-Y4 cells were pretreated with 10(-8) M 17-beta-estradiol. With 5, 10, and 20% CM, there was 46, 51, and 58% of inhibition. When the conditioned medium was treated with neutralizing antibody against transforming growth factor-beta (TGF-beta), the inhibitory effect was abolished. This suggests that osteocytes secrete significant amounts of TGF-beta, which inhibits bone resorption and is modulated by estrogen. RT-PCR and Western blot analysis show that in MLO-Y4 cells, the prevalent TGF-beta isoform is TGF-beta3. We conclude that osteocytes have an active, inhibitory role in the regulation of bone resorption. Our results further suggest a novel role for TGF-beta in the regulation of communication between different bone cells and suggest that at least part of the antiresorptive effect of estrogen in bone could be mediated via osteocytes.

Transforming growth factor-beta1 incorporated in calcium phosphate cement stimulates osteotransductivity in rat calvarial bone defects

Bone regeneration of the alveolar crest around dental implants is an important factor in the success of implant use. Calcium phosphate cement can be used as a bone substitute and applied clinically as a paste to fill micro- and macroscopic bone defects. We have shown earlier that the intermixing of the recombinant human transforming growth factor-beta1 (rhTGF-beta1) in hardening calcium phosphate cement stimulated osteoblastic differentiation of rat primary bone cells in vitro. The aim of the present study was to examine whether the similar enrichment with rhTGF-beta1 affects the replacement of calcium phosphate cement by bone (osteotransduction) in calvarial critical size defects (csd) of adult rats. Two bone defects of 5 mm diameter were created bilaterally in each skull of 10 adult male rats. Both defects were filled with 53 mg of calcium phosphate cement without rhTGF-beta1 (control) at one side, and with 10 or 20 ng rhTGF-beta1 at the other side. After 8 weeks, defects with surrounding skull were analysed histologically and histomorphometrically. The addition of rhTGF-beta1 in the cement increased the amount of bone in rat skull defects. This finding coincidences with our in vitro observations, that intermixing of rhTGF-beta1 in calcium phosphate cement stimulates bone cell differentiation. Addition of rhTGF-beta1 stimulated bone formation as indicated by an increased bone volume of 50% and an increased bone/cement contact of 65%, in comparison to control defects with cement without rhTGF-beta1. In addition, rhTGF-beta1 reduced the remaining volume of cement, by 11% at 10 ng rhTGF-beta1, and by 20% at 20 ng rhTGF-beta1 in the cement. Defect closure was not affected. We conclude that the intermixing of rhTGF-beta1 in a fast-setting calcium phosphate cement stimulates bone growth and the osteotransduction of the cement. For bone regeneration procedures around endosseous implants, calcium phosphate cement with rhTGF-beta1 might be an appropriate combination for early osseointegration and implant use.

Human breast-cancer cells stimulate the fusion, migration and resorptive activity of osteoclasts in bone explants

A central event in bone resorption is the recruitment of osteoclasts to future resorption sites. Breast-cancer cells invariably metastasise to the skeleton and induce extensive bone destruction by osteoclasts. However, our understanding of the mechanisms by which cancer cells interact with osteoclasts remains unclear. Consequently, we compared the effects of conditioned medium (CM) from 2 human breast-cancer cell lines, MB-MDA-231 and MCF-7, with those of a normal human breast epithelial cell line, HME, on osteoclastic fusion, resorptive activity and migration from the periosteum to the developing marrow cavity of fetal mouse metatarsals in culture. Osteoclastic resorptive activity was assessed by pre-labelling 17-day-old fetal metatarsal explants with 45Ca, whilst fusion and migration were monitored by histomorphometry and osteoclasts were identified by their tartrate-resistant acid phosphatase activity. CM from TPA-stimulated breast-cancer cell lines produced a significant increase in osteoclastic resorptive activity, whilst the normal breast cell line produced a minimal increase. The breast-cancer cell lines also stimulated osteoclastic fusion and migration in the metatarsal explants, but the normal breast cell line was without effect. The stimulatory effect of CM from MDA-MB-231 cells on osteoclastic fusion, but not migration, was partially inhibited by preventing prostaglandin and leukotriene synthesis by cells within the bone explants. In contrast, a synthetic matrix metalloproteinase (MMP) inhibitor, but not a cysteine proteinase inhibitor, prevented the migration of osteoclasts to the calcified centre of the metatarsal explants in response to CM from MDA-MB-231 cells. MDA-MB-231 cells also induced an increase in the expression of MMP-9 by migrating osteoclasts. Fractionation of the TPA-stimulated breast cancer cell CM established that the resorptive activity was associated with factors of m.w. >3 kDa. We determined by immuno-assay that human breast-cancer cells secrete parathyroid hormone-related protein (PTH-rP), tumour necrosis factor-alpha (TNF-alpha) and interleukins (ILs) 6 and 11. Neutralizing experiments with human antibodies to these cytokines established that PTH-rP and TNF-alpha production by MDA-MB-231 cells were responsible for mediating their effects on osteoclastic migration and ultimately bone resorption in the metatarsal explants.

Matrix metalloproteinase 9 and vascular endothelial growth factor are essential for osteoclast recruitment into developing long bones

Bone development requires the recruitment of osteoclast precursors from surrounding mesenchyme, thereby allowing the key events of bone growth such as marrow cavity formation, capillary invasion, and matrix remodeling. We demonstrate that mice deficient in gelatinase B/matrix metalloproteinase (MMP)-9 exhibit a delay in osteoclast recruitment. Histological analysis and specialized invasion and bone resorption models show that MMP-9 is specifically required for the invasion of osteoclasts and endothelial cells into the discontinuously mineralized hypertrophic cartilage that fills the core of the diaphysis. However, MMPs other than MMP-9 are required for the passage of the cells through unmineralized type I collagen of the nascent bone collar, and play a role in resorption of mineralized matrix. MMP-9 stimulates the solubilization of unmineralized cartilage by MMP-13, a collagenase highly expressed in hypertrophic cartilage before osteoclast invasion. Hypertrophic cartilage also expresses vascular endothelial growth factor (VEGF), which binds to extracellular matrix and is made bioavailable by MMP-9 (Bergers, G., R. Brekken, G. McMahon, T.H. Vu, T. Itoh, K. Tamaki, K. Tanzawa, P. Thorpe, S. Itohara, Z. Werb, and D. Hanahan. 2000. Nat. Cell Biol. 2:737-744). We show that VEGF is a chemoattractant for osteoclasts. Moreover, invasion of osteoclasts into the hypertrophic cartilage requires VEGF because it is inhibited by blocking VEGF function. These observations identify specific actions of MMP-9 and VEGF that are critical for early bone development.

Direct stimulation of osteoclastic bone resorption by bone morphogenetic protein (BMP)-2 and expression of BMP receptors in mature osteoclasts

Bone morphogenetic proteins (BMPs) play an important role in various kinds of pattern formation and organogenesis during vertebrate development. In the skeleton, BMPs induce the differentiation of cells of chondrocytic and osteoblastic cell lineage and enhance their function. However, the action of BMPs on osteoclastic bone resorption, a process essential for pathophysiological bone development and regeneration, is still controversial. In this study, we examine the direct effect of BMPs on osteoclastic bone-resorbing activity in a culture of highly purified rabbit mature osteoclasts. BMP-2 caused a dose- and time-dependent increase in bone resorption pits excavated by the isolated osteoclasts. BMP-4 also stimulated osteoclastic bone resorption. The increase in osteoclastic bone resorption induced by BMP-2 was abolished by the simultaneous addition of follistatin, a BMP/activin binding protein that negates their biological activity. Just as it increased bone resorption, BMP-2 also elevated the messenger RNA expressions of cathepsin K and carbonic anhydrase II, which are key enzymes for the degradation of organic and inorganic bone matrices, respectively. Type IA and II BMP receptors (BMPRs), and their downstream signal transduction molecules, Smad1 and Smad5, were expressed in isolated osteoclasts as well as in osteoblastic cells, whereas type IB BMPR was undetectable. BMPs directly stimulate mature osteoclast function probably mediated by BMPR-IA and BMPR-II and their downstream molecules expressed in osteoclasts. The results presented here expand our understanding of the multifunctional roles of BMPs in bone development.

Regulation of the differentiation and function of osteoclasts

The osteoclast is the cell that resorbs bone. It has been known for many years that its formation and function are regulated by cells of the osteoblastic lineage. Recently the molecular basis for this regulation was identified; osteoblastic cells induce osteoclastic differentiation and resorptive activity through expression of tumour necrosis factor (TNF) activation-induced cytokine (TRANCE) (also known as RANKL, ODF, OPGL, and TNFSF11), a novel membrane-inserted member of the TNF superfamily. Osteoclastic regulation is assisted through secretion of an inhibitor, osteoprotegerin (OPG) (OCIF, TNFRSF11B), a soluble (decoy) receptor for TRANCE. Osteoclast formation and survival also depend on and are substantially enhanced by transforming growth factor-beta (TGF-beta), which is abundant in bone matrix. Surprisingly, not only TRANCE but also TNF-alpha can induce osteoclast formation in vitro from bone marrow-derived mononuclear phagocytes, especially in the presence of TGF-beta. Whether or not TNF-alpha does the same in vivo, its ability to generate osteoclasts in vitro has significant implications regarding the nature of osteoclasts and their relationship to other mononuclear phagocytes, and a possible wider role for TRANCE in macrophage pathobiology. A hypothesis is presented in which the osteoclast is a mononuclear phagocyte directed towards a debriding function by TGF-beta, activated for this function by TRANCE, and induced to become specifically osteoclastic by the characteristics of the substrate or signals from bone cells that betoken such characteristics.

A role for TGFbeta(1) in osteoclast differentiation and survival

Recently, tumour necrosis factor-related activation-induced cytokine (TRANCE) was shown to be necessary for osteoclast formation. We now report that TGF(beta), a cytokine enriched in bone matrix, is also required. TGF(beta) not only powerfully synergized with TRANCE for induction of osteoclast-like cells (OCL) from bone marrow precursors and monocytes, but OCL formation was abolished by recombinant soluble TGF(beta) receptor II (TGF(beta)sRII). Preincubation in TGF(beta) was as effective as simultaneous incubation with TRANCE. TGF(beta)-preincubation enhanced OCL formation at least partly by preventing the development of resistance to OCL-induction that otherwise occurs when precursors are incubated in M-CSF. OCL formed in TRANCE also showed more rapid apoptosis than OCL in TRANCE plus TGF(beta). Like TGF(beta), incubation on bone matrix prolonged and enhanced the sensitivity of precursors to OCL-induction by TRANCE, and this was reversed by TGF(beta)sRII. Taken together, this data is compelling evidence for a model in which TGF(beta) in matrix or released from bone-lining or other cells maintains and enhances the osteoclast-forming potential of precursors as they migrate towards sites of cell-bound TRANCE. Thus, the specific circumstances necessary for osteoclast formation and survival are TRANCE expression on osteoblastic cells and TGF(beta) in bone.

Transforming growth factor-beta increases interleukin-6 transcripts in osteoblasts

Bone remodeling is regulated by local factors and cytokines. Among them, interleukin-6 (IL-6) plays a critical role in bone resorption, and its synthesis is stimulated by osteoresorptive factors. Transforming growth factor-beta (TGF-beta) is present in high amounts in the bone matrix and is a local regulator of bone formation. However, its role in bone resorption remains unclear. In this paper, we report that TGF-beta stimulates IL-6 transcripts in a time- and dose-dependent manner in primary rat osteoblasts isolated from 22-day-old calvariae (Ob cells). The TGF-beta effect on IL-6 mRNA levels does not require de novo protein synthesis because cycloheximide, a protein synthesis inhibitor, does not block the induction. The mechanisms of IL-6 stimulation by TGF-beta is at least partially transcriptional because TGF-beta induces IL-6 heterogenous nuclear RNA, and, to a lesser extent, IL-6 transcription rate as determined by a nuclear run-on assay. Transforming growth factor-beta upregulation of IL-6 may be critical in conditions of increased bone resorption, such as myeloma.

Inhibition of TGF-beta receptor signaling in osteoblasts leads to decreased bone remodeling and increased trabecular bone mass

Transforming growth factor-beta (TGF-beta) is abundant in bone matrix and has been shown to regulate the activity of osteoblasts and osteoclasts in vitro. To explore the role of endogenous TGF-(beta) in osteoblast function in vivo, we have inhibited osteoblastic responsiveness to TGF-beta in transgenic mice by expressing a cytoplasmically truncated type II TGF-beta receptor from the osteocalcin promoter. These transgenic mice develop an age-dependent increase in trabecular bone mass, which progresses up to the age of 6 months, due to an imbalance between bone formation and resorption during bone remodeling. Since the rate of osteoblastic bone formation was not altered, their increased trabecular bone mass is likely due to decreased bone resorption by osteoclasts. Accordingly, direct evidence of reduced osteoclast activity was found in transgenic mouse skulls, which had less cavitation and fewer mature osteoclasts relative to skulls of wild-type mice. These bone remodeling defects resulted in altered biomechanical properties. The femurs of transgenic mice were tougher, and their vertebral bodies were stiffer and stronger than those of wild-type mice. Lastly, osteocyte density was decreased in transgenic mice, suggesting that TGF-beta signaling in osteoblasts is required for normal osteoblast differentiation in vivo. Our results demonstrate that endogenous TGF-beta acts directly on osteoblasts to regulate bone remodeling, structure and biomechanical properties.

Cell biology of the osteoclast

The osteoclast is a hematopoietic cell derived from CFU-GM and branches from the monocyte-macrophage lineage early during the differentiation process. The marrow microenvironment appears critical for osteoclast formation due to production of RANK ligand, a recently described osteoclast differentiation factor, by marrow stromal cells in response to a variety of osteotropic factors. In addition, factors such as osteoprotegerin, a newly described inhibitor of osteoclast formation, as well as secretory products produced by the osteoclast itself and other cells in the marrow enhance or inhibit osteoclast formation. The identification of the role of oncogenes such as c-fos and pp60 c-src in osteoclast differentiation and bone resorption have provided important insights in the regulation of normal osteoclast activity. Current research is beginning to delineate the signaling pathways involved in osteoclastic bone resorption and osteoclast formation in response to cytokines and hormones. The recent development of osteoclast cell lines may make it possible for major advances to our understanding of the biology of the osteoclast to be realized in the near future.

Bone marrow cells produce soluble factors that inhibit osteoclast activity

Cytokines that stimulate bone resorption are produced by cells found in bone marrow. However, marrow cells produce multiple factors, some of which may be inhibitors of osteoclast differentiation or activity. Thus, it is not possible to predict a priori whether the mixture of factors produced by marrow cells will have a net stimulatory or inhibitory effect on bone resorption. In this study, we showed that the net effect of whole marrow is to inhibit osteoclast activity induced by parathyroid hormone. Fractionation of the marrow revealed that the inhibitory activity was in the marrow fluid. However, conditioned media obtained from marrow cell cultures also inhibited osteoclast activity. Thus, it is likely that the inhibitory factors are produced in vivo by cells residing in the marrow. These inhibitory factors may represent a physiological regulatory process that plays an important role in maintaining the balance between bone resorption and formation. Because we have previously shown that interleukin-6 is one of the cytokines that parathyroid hormone induces in osteoblastic cells to stimulate osteoclast activity, one potential mechanism by which the marrow-derived inhibitory factors might act is by preventing this production of interleukin-6. However, we found that the marrow cell-conditioned media do not inhibit the production or activity of interleukin-6. Thus, the inhibitory factors appear to block osteoclast activity through a mechanism that does not involve interleukin-6. Taken together, these results demonstrate the importance of factors that inhibit bone resorption and emphasize that the presence of cytokines that stimulate bone resorption in conditions such as osteoporosis and orthopaedic implant loosening should be interpreted with caution unless evidence exists demonstrating their functional importance.

Later orthodontic appliance reactivation stimulates immediate appearance of osteoclasts and linear tooth movement

BACKGROUND

Delays in the appearance of osteoclasts at compression sites occur after orthodontic appliance reactivation, when this is done during both the period of osteoclast recruitment and the peak expansion in the osteoclast population. This experiment examines osteoclasts and tooth movement in alveolar bone after appliance reactivation coinciding with alveolar bone formation and the time when reactivation osteoclasts first appear (ie, 10 days after initial appliance activation).

METHODS

Bilateral orthodontic appliances were activated to mesially tip maxillary molars with 40 cN in 144 rats. After 10 days, all rats were randomized into two groups of 72. Group I had appliances reactivated in precisely the same manner as the first activation. Group II had appliances sham-reactivated. Nine to 12 rats were then sacrificed at 1, 3, 5, 7, 10, and 14 days in both groups (eg, day 1 represents an interval of 11 days after the first appliance activation and 1 day after either sham or real reactivation). Orthodontic movement was measured cephalometrically; changes in osteoclasts and root resorption were assessed at both compression and tension sites histomorphometrically.

RESULTS

Teeth in the reactivated group (Group I) displayed linear tooth movement (62.6 micrometers/day), and 0.9 mm tooth movement by day 10. Significant increases in osteoclast numbers, osteoclast surface percentage, and surface per individual osteoclast were evident in these animals by 1 day postreactivation (P <.01). Significant treatment-related increases in root resorption were not evident at compression sites at any time.

CONCLUSIONS

These findings indicate that, after appliance reactivation during the time when reactivation osteoclasts appear, a second cohort of osteoclasts can be recruited immediately, along with immediate and substantial tooth movement and no greater risk of root resorption.

Transforming growth factor-beta stimulates the production of osteoprotegerin/osteoclastogenesis inhibitory factor by bone marrow stromal cells

Osteoprotegerin (OPG)/osteoclastogenesis inhibitory factor (OCIF) is a recently identified cytokine that belongs to the tumor necrosis factor receptor superfamily and regulates bone mass by inhibiting osteoclastic bone resorption. The present study was undertaken to determine whether OPG/OCIF is produced in bone microenvironment and how the expression is regulated. A transcript for OPG/OCIF at 3.1 kilobases was detected in bone marrow stromal cells (ST2 and MC3T3-G2/PA6) as well as in osteoblastic cells (MC3T3-E1). Transforming growth factor-beta1 (TGF-beta1) markedly increased the steady-state level of OPG/OCIF mRNA in a dose-dependent manner, while TGF-beta1 suppressed the mRNA expression of tumor necrosis factor-related activation-induced cytokine (TRANCE)/receptor activator of NF-kappaB ligand (RANKL), a positive regulator of osteoclastogenesis to which OPG/OCIF binds. The effect of TGF-beta1 on the expression of OPG/OCIF mRNA was transient, with a peak level at 3-6 h. The up-regulation of OPG/OCIF mRNA by TGF-beta1 in ST2 cells did not require de novo protein synthesis and involved both a transcriptional and a post-transcriptional mechanism. Western blot analysis and an enzyme-linked immunosorbent assay revealed that TGF-beta1 significantly increased the secretion of OPG/OCIF protein by ST2 cells at 6-24 h. In murine bone marrow cultures, TGF-beta1 markedly inhibited the formation of tartrate-resistant acid phosphatase-positive multinucleated osteoclast-like cells in the presence of 1,25-dihydroxyvitamin D3, whose effect was significantly reversed by a neutralizing antibody against OPG/OCIF. These results suggest that TGF-beta1 negatively regulates osteoclastogenesis, at least in part, through the induction of OPG/OCIF by bone marrow stromal cells and that the balance between OPG/OCIF and TRANCE/RANKL in local environment may be an important determinant of osteoclastic bone resorption.

Response of normal and osteoporotic human bone cells to mechanical stress in vitro

Bone adapts to mechanical stress, and bone cell cultures from animal origin have been shown to be highly sensitive to mechanical stress in vitro. In this study, we tested whether bone cell cultures from human bone biopsies respond to stress in a similar manner as animal bone cells and whether bone cells from osteoporotic patients respond similarly to nonosteoporotic donors. Bone cell cultures were obtained as outgrowth from collagenase-stripped trabecular bone fragments from 17 nonosteoporotic donors between 7 and 77 yr of age and from 6 osteoporotic donors between 42 and 72 yr of age. After passage, the cells were mechanically stressed by treatment with pulsating fluid flow (PFF; 0.7 +/- 0.03 Pa at 5 Hz for 1 h) to mimic the stress-driven flow of interstitial fluid through the bone canaliculi, which is likely the stimulus for mechanosensation in bone in vivo. Similar to earlier studies in rodent and chicken bone cells, the bone cells from nonosteoporotic donors responded to PFF with enhanced release of prostaglandin E2 (PGE2) and nitric oxide as well as a reduced release of transforming growth factor-beta (TGF-beta). The upregulation of PGE2 but not the other responses continued for 24 h after 1 h of PFF treatment. The bone cells from osteoporotic donors responded in a similar manner as the nonosteoporotic donors except for the long-term PGE2 release. The PFF-mediated upregulation of PGE2 release during 24 h of postincubation after 1 h of PFF was significantly reduced in osteoporotic patients compared with six age-matched controls as well as with the whole nonosteoporotic group. These results indicate that enhanced release of PGE2 and nitric oxide, as well as reduced release of TGF-beta, is a characteristic response of human bone cells to fluid shear stress, similar to animal bone cells. The results also suggest that bone cells from osteoporotic patients may be impaired in their long-term response to mechanical stress.

Effect of orthodontic appliance reactivation during the period of peak expansion in the osteoclast population

BACKGROUND

Delays in the appearance ofosteoclasts at compression sites occur following orthodontic appliance reactivation when this is done during the period of osteoclast recruitment. This study examined changes in alveolar bone after appliance reactivation at a time coinciding with the peak expansion of the osteoclast population following the first appliance activation.

METHODS

Orthodontic appliances were activated with 40 g on maxillary molars followed by a reactivation with the same force after 4 days in one group and sham reactivation in the other. Rats were killed at 0, 1, 3, 6, and 10 days thereafter. Orthodontic movement was measured cephalometrically. TRAP and interleukin-1 alpha (IL-1alpha) were measured biochemically, and changes in osteoclasts and root resorption were assessed at both compression and tension sites histomorphometrically.

RESULTS

Teeth in the reactivated group displayed more initial displacement than controls but no more tooth movement 10 days following appliance reactivation. Also, increases in osteoclast numbers and surface percent, as well as alveolar bone Tartrate-resistant acid phosphatase (TRAP), became evident in the treated animals only 10 days after reactivation. However, IL-1alpha was elevated in alveolar bone within 1 hr following appliance reactivation but returned to baseline by day 1. There were no treatment-related difference in nuclear number per osteoclast or trabecular surface per osteoclast. Significant treatment-related increases in root resorption were evident at compression sites by day 10.

CONCLUSIONS

These findings indicate that after appliance reactivation during the height of osteoclastic stimulation, a second cohort of osteoclasts can be recruited, but only after a delay of several days. This delay is not due to a failure to produce IL-1alpha in the tissues.

Heterogeneity among cells that express osteoclast-associated genes in developing bone

In the present study, we characterized the phenotype of cells in the osteoclast lineage by in situ hybridization, using antisense complementary RNA probes that encode three genes typically expressed by osteoclasts, tartrate-resistant acid phosphatase (TRAP), type IV collagenase (matrix metalloproteinase-9), and c-fms, the receptor for macrophage colony-stimulating factor. By using complementary RNA probes labeled with 35S, digoxygenin, or a combination of the two labeling methods (dual labeling in situ hybridization), we found that each of these genes exhibited a distinct expression pattern during early stages of endochondral bone development [embryonic day 15 (ED15) to ED17] in fetal mouse hind limbs. Type IV collagenase messenger RNA (mRNA) was first expressed in or just outside of the cellular layers that define perichondrium/periosteum, earlier than transcripts for TRAP or c-fms appeared at the same sites (ED15). Although transcripts for TRAP and c-fms colocalized within the skeleton, c-fms was also found in surrounding soft tissue, whereas TRAP mRNA was never detected outside the skeleton (ED16). Type IV collagenase mRNA was uniquely distributed at the chondro-osseous border, being distinct from the distribution of TRAP or c-fms (ED17). At later stages of skeletal development (ED18 to 15-day-old postnatal bone), however, there was more overlap among TRAP, type IV collagenase, and c-fms mRNAs in cells throughout bone, except at the chondro-osseous junction, where type IV collagenase continued to be uniquely localized to some cells at all developmental stages. Whereas the levels of type IV collagenase mRNA expression was most intense at the chondro-osseous margin, the levels of c-fms and TRAP mRNA expression appeared to be more uniform throughout the developing bone. The results indicate that there is considerable heterogeneity among cells expressing osteoclast-associated genes, particularly during early stages of endochondral bone development, but that this difference becomes less pronounced later in the more mature skeleton. Distinct expression patterns of these markers may represent different stages of osteoclastogenesis. Alternatively, type IV collagenase-positive and TRAP/c-fms-positive cells may represent distinct subpopulations of cells of the osteoclast lineage.

Histomorphometric and biochemical study of osteoclasts at orthodontic compression sites in the rat during indomethacin inhibition

Prostaglandins affect the number of osteoclasts at compression sites in orthodontic tooth movement. They may also have a role in tooth movement and influence the extent of root resorption. The purpose was to examine the effect of indomethacin on the activity of resident osteoclasts, recruitment of new osteoclasts and root resorption at orthodontic compression sites. Two separate populations of osteoclasts were studied: those resident at the sites after initial appliance activation and those recruited by a subsequent activation. Orthodontic appliances were activated to provide mesially directed forces of 40 g on the maxillary molars of rats. The appliances were activated with the same force after 4 days. The rats were killed at 1, 3, 6 and 10 days after initial activation. Half of the rats were injected with indomethacin. Tooth movement was measured cephalometrically; osteoclast numbers, sizes, numbers of nuclei per osteoclast and root resorption were assessed histomorphometrically; tartrate-resistant acid phosphatase (TRAP) in alveolar bone was measured biochemically. Indomethacin inhibited both initial tooth displacement and that following the delay. It also reduced the increase in osteoclast numbers, total osteoclast surface and alveolar bone TRAP at day 10. It had no effect on the surface area of each individual osteoclast or number of nuclei in each osteoclast. Root resorption increased in both groups but it was enhanced at day 10 in the indomethacin group. These data suggest that orthodontic tooth movement after appliance activation requires the recruitment of osteoclasts to sites of compression and that this is indomethacin-sensitive. Furthermore, indomethacin enhances root resorption at compression sites 10 days after appliance reactivation.

A sequence variation: 713-8delC in the transforming growth factor-beta 1 gene has higher prevalence in osteoporotic women than in normal women and is associated with very low bone mass in osteoporotic women and increased bone turnover in both osteoporotic and normal women

Bone mass is partly genetically determined. The genes involved are, however, still largely unknown. Transforming growth factor-beta 1 (TGF-beta 1) is considered a putative regulator of osteoclastic-osteoblastic interaction (coupling). The aim of the present study was therefore to examine whether possible variants of the TGF-beta 1 gene are related to bone mass and osteoporosis. We examined 161 osteoporotic women (at least one low energy spinal fracture) and 131 normal women. We investigated sequence variations in the TGF-beta 1 gene using the single-stranded conformation polymorphism (SSCP) technique combined with DNA sequencing. Seven patients were heterozygous for a cytosine to thymidine base substitution at position 76 in exon 5 (C788-T) (corresponding to position 788 in the TGF-beta 1 cDNA), resulting in a threonine to isoleucine amino acid shift at position 263 in the TGF-beta 1 propeptide (Thr263-Ile). Ten other patients had a one base deletion in the intron sequence 8 bases prior to exon 5 (713-8delC), which could influence splicing. Five normal women exhibited the C788-T sequence variant, and two the 713-8delC. The prevalence of 713-8delC was significantly higher in the osteoporotic group (chi 2 = 4.02, p < 0.05). Osteoporotic patients with the 713-8delC variant had increased levels of bone alkaline phosphatase (p < 0.05). If the osteoporotic patients with a z score of the lumbar spine below -1 were examined separately, we found increased serum levels of bone alkaline phosphatase (p < 0.05), increased urinary excretion of hydroxyproline (p < 0.05), and reduced bone mass of the lumbar spine (p < 0.05) in patients with 713-8delC. No correlation to bone mass was demonstrated in the normal women, but 713-8delC was associated with increased serum levels of bone alkaline phosphatase (p < 0.05). The sequence variation, 713-8delC, in the TGF-beta 1 gene is more frequent in patients with osteoporosis compared to normal controls. The 713-8delC variant seems to be associated with very low bone mass in osteoporotic women with low bone mass and increased bone turnover in both osteoporotic and normal women.

Transforming growth factor-beta stimulates bone resorption in neonatal mouse calvariae by a prostaglandin-unrelated but cell proliferation-dependent pathway

The relationships between bone resorption, prostanoid formation, and cell proliferation in cultured neonatal mouse calvariae stimulated with transforming growth factor-beta (TGF-beta) have been examined. Bone resorption was assessed by analyzing the mobilization of minerals (45Ca, Ca2+., Pi) and the release of 3H from bones prelabeled with [3H]proline. Prostanoid formation was determined by analyzing the amounts of prostaglandin E2 (PGE2) and 6-keto-prostaglandin F1 alpha (the stable breakdown product of PGI2) in culture media. Purified porcine TGF-beta 1 and recombinant human TGF-beta 2 stimulated the release of 45Ca and the formation of prostanoids. The effects were time and dose dependent. The concentrations of TGF-beta 1 and TGF-beta 2 causing half maximal stimulation of 45Ca release were 1 and 0.1 ng/ml, respectively. TGF-beta 1 also enhanced the release of 3H from [3H]proline labeled bones and the mobilization of Ca2+ and Pi from unlabeled bones, as well as the release of lysosomal enzymes (beta-N-acetylglucosaminidase). The degree of stimulation of mineral mobilization and matrix degradation was less than that obtained in bones stimulated with parathyroid hormone or 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). TGF-beta 1-induced stimulation of 45Ca release was inhibited by calcitonin, acetazolamide, and the biphosphonate AHPrBP, three different osteoclast inhibitors. In contrast to the escape from calcitonin-induced inhibition seen in parathyroid hormone (PTH)-stimulated bones, the inhibitory effect of calcitonin in TGF-beta 1-stimulated bones persisted in long-term cultures (144 h). The stimulatory effect of TGF-beta 1 was inhibited by anti-TGF-beta 1 and by gamma-interferon (1000 U/ml). Indomethacin (1 microM), flurbiprofen (1 microM), and meclofenamic acid (1 microM) completely abolished the stimulatory effect of TGF-beta 1 on PGE2 and 6-keto-PGF 1 alpha formation without affecting TGF-beta 1-induced stimulation of 45Ca release. Similarly, the stimulatory effect of TGF-beta 2 on 45Ca release was unaffected by indomethacin. In bones in which prostaglandin formation was abolished by indomethacin, a 45Ca release response to TGF-beta 1 was obtained at 12 h. The mitotic inhibitor hydroxyurea inhibited TGF-beta 1 but not PTH-induced 45Ca release. These data demonstrate that TGF-beta 1 and TGF-beta 2 have the capacity to stimulate bone resorption and prostanoid formation in neonatal mouse calvariae, but that the effect of TGF-beta on bone resorption is unrelated to prostanoid formation. In addition, it is shown that bone resorption stimulated by TGF-beta is dependent on cell replication.

Bone resorption and response to calcium-regulating hormones in the absence of tissue or urokinase plasminogen activator or of their type 1 inhibitor

Plasminogen activators (PA) are implicated in cell migration and tissue remodeling, two components of the bone resorption processes. Using mice with inactivated tissue PA (tPA), urokinase PA (uPA), or type 1 PA inhibitor (PAI-1) genes, we evaluated whether these processes, or their stimulation by parathyroid hormone (PTH) or 1,25-dihydroxyvitamin (1,25[OH]2D3) are dependent on these genes. Two culture models were used, one involving 19-day fetal calvariae, to evaluate the direct resorptive activity of osteoclasis, and the other involving 45Ca-labeled 17-day fetal metatarsals, in which this activity depends on preliminary (pre)osteoclast migration. PTH similarly increased (about 10-fold) PA activity in calvariae from wild-type tPA+/+ and uPA+/+ or deficient uPA-/- and PAI-/- mice; it affected only tPA, not uPA. In tPA-/- bones, the low PA levels, due to uPA, were not influenced by PTH. Calcitonin did not affect PA responses to PTH. No differences were observed between tPA+/+, tPA-/-, uPA+/+, and uPA-/- calvariae for any parameter related to bone resorption (development of lacunae, release of calcium and lysosomal enzymes, accumulation of collagenase, loss of hydroxyproline), indicating similar responses to PTH or calcitonin. The progressive 45Ca release was largely similar in cultures of tPA+/+, tPA-/-, uPA+/+, uPA-/-, PAI+/+, or PAI-/- metatarsals and it was similarly enhanced by PTH or 1,25(OH)2D3. However, uPA-/- metatarsals released 45Ca at a slower rate at the beginning of the cultures, suggesting an impaired recruitment of the (pre)osteoclasts, which migrate at that time from the periosteum into the calcified cartilage. Thus, it appears that the direct resorptive activity of the osteoclasts does not necessitate the presence of either tPA or uPA, but uPA is likely to facilitate the migration of the (pre)osteoclasts toward the mineralized surfaces. Although considerably enhanced by PTH, tPA does not mediate the actions of PTH (nor of 1,25[OH]2D3) evaluated in these models.

Prostaglandin mediated modulation of transforming growth factor-beta metabolism in primary mouse osteoblastic cells in vitro

Prostaglandins and transforming growth factor-beta (TGF-beta) are both important local regulators of bone metabolism, but their actions on bone are complex. Prostaglandins mediate bone loss due to immobilization, but prostaglandin E2 (PGE2) treatment stimulates bone formation in vivo. TGF-beta may have both anabolic and catabolic effects on bone in vitro. In this study, we tested the effects of PGE2 on TGF-beta release and on TGF-beta messenger RNA (mRNA) levels in neonatal mouse calvarial cell cultures. We also examined the relationship between endogenous prostaglandin production as a result of mechanical stress and the release of TGF-beta. Addition of PGE2 (10(-8)-10(-6)M) to the culture medium stimulated the release of TGF-beta peptide (active plus latent) after 24 and 48 h in a dose-related manner. This upregulation was paralleled by an increased expression of TGF-beta mRNA levels. Mechanical stimulation by 1 h treatment with pulsating fluid flow (producing a shear stress of 0.5 +/- 0.02 Pa at 5 Hz) resulted 1 h posttreatment in increased production of PGE2, prostaglandin l2 (PGI2), and prostaglandin F2a. In addition, the release of TGF-beta activity but not TGF-beta peptide was decreased 24 h after PFF treatment. Addition of indomethacin, which blocks endogenous prostaglandin production, neutralized the effect of PFF treatment on TGF-beta activity, indicating that the effect of stress was mediated by endogenous prostaglandins. These results suggest that PGE2 and other prostaglandins (probably PGI2 and/or PGF2a) have opposite effects on TGF-beta metabolism in bone cells, as PGE2 upregulates TGF-beta expression and synthesis while other prostaglandins downregulate TGF-beta activation.

Identification of IGF-I in the calvarial suture of young rats: histochemical analysis of the cranial sagittal sutures in a hyperthyroid rat model

Premature closure of cranial sutures has been known as one of the complications of juvenile or congenital hyperthyroidism. Thyroid hormone is an anabolic agent for bone formation in the early stages of childhood development. In children, excess thyroid hormone acts as an acceleration factor for the skeletal bone, whereas in adult hyperthyroidism, it causes bone mineral loss due to the high turnover rate of bone formation and consequently bone resorption. In addition, there are numerous literature descriptions concerning the interactions among bone metabolism, hormones, and growth factors, among which insulin-like growth factor I (IGF-I) is the most abundantly found growth factor in osteoblasts and in bone models in vivo. We therefore investigated whether or not the cranial sutures show accelerated closure and how the local growth factors or cytokines participate and function in local bone metabolism after administration of exogenous excess thyroid hormone in a rat model. A total of 60 female Wistar rats, aged 10 days, were divided into two groups, the triiodothyronine (T3)-treated group (n = 30, T3 0.1 microgram/gm of body weight per day) and the control group (n = 30, saline vehicle only), and were maintained and subsequently sacrificed at 15, 30, and 60 days. The parameters of cranial width derived from the morphologic measurements of the skull indicated that the lambda-asterion distance at 30 days and the pterion-bregma distance at 60 days in the T3-treated group were significantly decreased compared with those of the control group. Furthermore, the fluorescent histologic findings showed fluorescent labeling with no interruption along the suture edges, suggesting continuous bone formation, and displayed narrowing of the suture gap of the sagittal suture in the T3-treated group. Tartrate resistant acid phosphatase staining showed very little osteoclastic activity in the sagittal suture, especially in the T3-treated group. The intensity of immunohistochemical staining of IGF-I was markedly increased in the suture margins of the T3-treated group. There were no significant differences observed either in the skull base measurements or in the histologic and histochemical findings of the skull base or the coronal suture between the groups. More significantly, excess administration of thyroid hormone enhanced the cranial sagittal suture closure; therefore, it was proposed that local IGF-I plays an important role in sagittal sutural bone formation.

Increased expression of TGF-beta 2 in osteoblasts results in an osteoporosis-like phenotype

The development of the skeleton requires the coordinated activities of bone-forming osteoblasts and bone-resorbing osteoclasts. The activities of these two cell types are likely to be regulated by TGF-beta, which is abundant in bone matrix. We have used transgenic mice to evaluate the role of TGF-beta 2 in bone development and turnover. Osteoblast-specific overexpression of TGF-beta 2 from the osteocalcin promoter resulted in progressive bone loss associated with increases in osteoblastic matrix deposition and osteoclastic bone resorption. This phenotype closely resembles the bone abnormalities seen in human hyperparathyroidism and osteoporosis. Furthermore, a high level of TGF-beta 2 overexpression resulted in defective bone mineralization and severe hypoplasia of the clavicles, a hallmark of the developmental disease cleidocranial dysplasia. Our results suggest that TGF-beta 2 functions as a local positive regulator of bone remodeling and that alterations in TGF-beta 2 synthesis by bone cells, or in their responsiveness to TGF-beta 2, may contribute to the pathogenesis of metabolic bone disease.

Cytokine regulation of bone cell differentiation

Systemic hormones and cytokines play important roles in regulating both osteoblast and osteoclast activity. These cytokines can have either positive or negative effects on the growth and differentiation of bone cells. These effects appear to be dependent on the model systems use to assess them, as well as the species tested. In the near future, other autocrine-paracrine factors will be identified that enhance osteoblast and osteoclast activity, and model systems should be available to further delineate their effects on cells in the osteoblast lineage. Use of transgenic mice with genes targeted to the osteoblast and osteoclast may further reveal the mechanisms responsible for the growth and differentiation of these cells, as well as produce immortalized cell lines that more accurately reflect the cell biology of the osteoclast and osteoblast in vivo.

Matrix metalloproteinases are obligatory for the migration of preosteoclasts to the developing marrow cavity of primitive long bones

A key event in bone resorption is the recruitment of osteoclasts to future resorption sites. We follow here the migration of preosteoclasts from the periosteum to the developing marrow cavity of fetal mouse metatarsals in culture, and investigate the role of proteinases and demineralization in this migration. Our approach consisted in testing inhibitors of proteinases and demineralization on the migration kinetics. Migration was monitored by histomorphometry and the (pre)osteoclasts were identified by their tartrate resistant acid phosphatase (TRAP) activity. At the time of explantation, TRAP+ cells (all mononucleated) are detected only in the periosteum, and the core of the diaphysis (future marrow cavity) consist of calcified cartilage. Upon culture, TRAP+ cells (differentiating progressively into multinucleated osteoclasts) migrate through a seam of osteoid and a very thin and discontinuous layer of mineral, invade the calcified cartilage and transform it into a “marrow' cavity; despite the passage of maturing osteoclasts, the osteoid develops into a bone collar. The migration of TRAP+ cells is completely prevented by matrix metalloproteinase (MMP) inhibitors, but not by a cysteine proteinase inhibitor, an inhibitor of carbonic anhydrase, or a bisphosphonate. The latter three drugs inhibit, however, the resorptive activity of mature osteoclasts at least as efficiently as do the MMP inhibitors, as assessed in cultures of calvariae and radii. Furthermore, in situ hybridizations reveal the expression of 2 MMPs, gelatinase B (MMP-9 or 92 kDa type IV collagenase) in (pre)osteoclasts, and interstitial collagenase (MMP-13) in hypertrophic chondrocytes. It is concluded that only MMPs appear obligatory for the migration of (pre)osteoclasts, and that this role is distinct from the one MMPs may play in the subosteoclastic resorption compartment. We propose that this new role of MMPs is a major component of the mechanism that determines where and when the osteoclasts will attack the bone.

Cranial sutures require tissue interactions with dura mater to resist osseous obliteration in vitro

A chemically defined serum-free medium, which supports the development of bones and fibrous tissues of rat calvaria from nonmineralized mesenchymal precursor tissues, was employed to investigate tissue interactions between the dura matter and overlying tissues. Fetal calvarial rudiments from stages prior to bone and suture morphogenesis (fetal days 19 and 20) and neonatal calvarial rudiments with formed sutures (day 1) were cultured with and without associated dura mater. Removal of calvaria for in vitro culture allowed the examination of suture morphogenesis in the absence of tensional forces exerted on the sutures via fiber tracts in the dura mater originating in the cranial base. Ossification of frontal and parietal bones proceeded in a fashion comparable to development in vivo, but the cranial (coronal) sutures--primary sites for subsequent skull growth--were obliterated by osseous tissue union in the absence of dura mater. Bony fusion did not occur when rudiments were cocultured with dura mater on the opposite sides of 0.45 microns polycarbonate transwell filters, suggesting that the influence of dura mater on sutural obliteration was mediated by soluble factors rather than cell-cell or cell-matrix interactions. These results indicate that cell signaling mechanisms rather than biomechanical tensional forces are required for morphogenesis of the calvaria.