Glucocorticoids promote development of the osteoblast phenotype by selectively modulating expression of cell growth and differentiation associated genes

Pathogenic mechanisms of glucocorticoid-induced osteoporosis

Glucocorticoid (GC) is one of the most prescribed medicines to treat various inflammatory and autoimmune diseases. However, high doses and long-term use of GCs lead to multiple adverse effects, particularly glucocorticoid-induced osteoporosis (GIO). Excessive GCs exert detrimental effects on bone cells, including osteoblasts, osteoclasts, and osteocytes, leading to impaired bone formation and resorption. The actions of exogenous GCs are considered to be strongly cell-type and dose dependent. GC excess inhibits the proliferation and differentiation of osteoblasts and enhances the apoptosis of osteoblasts and osteocytes, eventually contributing to reduced bone formation. Effects of GC excess on osteoclasts mainly include enhanced osteoclastogenesis, increased lifespan and number of mature osteoclasts, and diminished osteoclast apoptosis, which result in increased bone resorption. Furthermore, GCs have an impact on the secretion of bone cells, subsequently disturbing the process of osteoblastogenesis and osteoclastogenesis. This review provides timely update and summary of recent discoveries in the field of GIO, with a particular focus on the effects of exogenous GCs on bone cells and the crosstalk among them under GC excess.

The hidden link between circadian entropy and mental health disorders

The high overlapping nature of various features across multiple mental health disorders suggests the existence of common psychopathology factor(s) (p-factors) that mediate similar phenotypic presentations across distinct but relatable disorders. In this perspective, we argue that circadian rhythm disruption (CRD) is a common underlying p-factor that bridges across mental health disorders within their age and sex contexts. We present and analyze evidence from the literature for the critical roles circadian rhythmicity plays in regulating mental, emotional, and behavioral functions throughout the lifespan. A review of the literature shows that coarse CRD, such as sleep disruption, is prevalent in all mental health disorders at the level of etiological and pathophysiological mechanisms and clinical phenotypical manifestations. Finally, we discuss the subtle interplay of CRD with sex in relation to these disorders across different stages of life. Our perspective highlights the need to shift investigations towards molecular levels, for instance, by using spatiotemporal circadian "omic" studies in animal models to identify the complex and causal relationships between CRD and mental health disorders.

Effect of dexamethasone on the growth and differentiation of osteoblast-like cells derived from the human alveolar bone

Objectives

This study aimed to investigate the effect of dexamethasone on the growth and differentiation of osteoblast-like cells derived from the human alveolar bone.

Methods

Bone particles were collected from patients during implant-site preparation. The samples were cultured in a growth medium, and the cells that propagated after two–three weeks were cultured in three types of culture media: group 1, normal medium; group 2, osteogenic medium without dexamethasone; and group 3, osteogenic medium with dexamethasone—for zero, four, seven, and 20 days. DNA and alkaline phosphatase (ALP) measurements and alizarin red/toluidine blue staining were performed.

Results

DNA levels were significantly higher in group 2 than in group 1 on day 7 (p < 0.001) and in group 3 on days 4, 7, and 20 (p < 0.041, p < 0.006, and p < 0.001, respectively). Further, total ALP levels were significantly higher in group 3 than in groups 1 on day 20 (p < 0.023). A greater amount of matrix mineralisation was observed in group 3 than in groups 1 and 2.

Conclusions

Human alveolar bone cells exhibit improved osteogenic efficacy in terms of osteogenic differentiation when cultured in the presence of dexamethasone. The cell number (total DNA content) decreased in the presence of dexamethasone; however, an increased differentiation of osteoblast-like cells was observed.

Bad to the Bone: The Effects of Therapeutic Glucocorticoids on Osteoblasts and Osteocytes

Despite the continued development of specialized immunosuppressive therapies in the form of monoclonal antibodies, glucocorticoids remain a mainstay in the treatment of rheumatological and auto-inflammatory disorders. Therapeutic glucocorticoids are unmatched in the breadth of their immunosuppressive properties and deliver their anti-inflammatory effects at unparalleled speed. However, long-term exposure to therapeutic doses of glucocorticoids decreases bone mass and increases the risk of fractures - particularly in the spine - thus limiting their clinical use. Due to the abundant expression of glucocorticoid receptors across all skeletal cell populations and their respective progenitors, therapeutic glucocorticoids affect skeletal quality through a plethora of cellular targets and molecular mechanisms. However, recent evidence from rodent studies, supported by clinical data, highlights the considerable role of cells of the osteoblast lineage in the pathogenesis of glucocorticoid-induced osteoporosis: it is now appreciated that cells of the osteoblast lineage are key targets of therapeutic glucocorticoids and have an outsized role in mediating their undesirable skeletal effects. As part of this article, we review the molecular mechanisms underpinning the detrimental effects of supraphysiological levels of glucocorticoids on cells of the osteoblast lineage including osteocytes and highlight the clinical implications of recent discoveries in the field.

Lycii radicis cortex inhibits glucocorticoid‑induced bone loss by downregulating Runx2 and BMP‑2 expression

Lycii radicis cortex (LRC) has been used to regulate high blood pressure, body temperature, pain and bone disorders in East Asia. Glucocorticoids (GCs), also known as steroids, are potent immunity regulators widely used in the treatment of inflammatory diseases. However, despite their effectiveness, GC usage is strictly controlled due to severe side‑effects, such as osteoporosis. However, further research is required as to date, at least to the best of our knowledge, there is no appropriate model to overcome secondary osteoporosis as a side‑effect of GC use. Thus, the aim of the present study was to establish an experimental model of osteoporosis induced by GC. Furthermore, the present study aimed to establish the research methodology for medical evaluations of the effectiveness and side‑effects of GCs. A secondary osteoporosis animal model was established, and the animals were divided into two groups as follows: The allergic contact dermatitis (ACD)‑induced group and the non‑ACD‑induced group. In the ACD‑induced group, a GC topical application group was compared with a GC subcutaneous injection group. The results revealed that the presence of ACD affected the induction of GC‑mediated osteoporosis. Therefore, the group exhibiting induced ACD that was treated with a topical application of GC was selected for examining the side‑effects of GCs. The effects of LRC on secondary osteoporosis were confirmed in vivo and in vitro. The results indicated that LRC regulated dexamethasone‑induced osteoblast apoptotic markers, including caspase‑6, caspase‑9, X‑linked inhibitor of apoptosis, apoptosis inhibitor 1 and apoptosis inhibitor 2, and increased the expression of osteoblast differentiation‑related genes, such as Runt‑related transcription factor 2 and bone morphogenetic protein 2 in the MC3T3E‑1 cell line. LRC also significantly reduced GC‑induced osteoporosis and exerted anti‑inflammatory effects in vivo. In addition, LRC inhibited the reduction of calbindin‑D28k in the kidney. Overall, the results of the present study suggest that the use of LRC alleviates GC‑induced secondary osteoporosis.

Optimizing In Vitro Osteogenesis in Canine Autologous and Induced Pluripotent Stem Cell-Derived Mesenchymal Stromal Cells with Dexamethasone and BMP-2

A growing body of work suggests that canine mesenchymal stromal cells (cMSCs) require additional agonists such as bone morphogenic protein-2 (BMP-2) for consistent in vitro osteogenic differentiation. BMP-2 is costly and may challenge the translational relevance of the canine model. Dexamethasone enhances osteogenic differentiation of human MSCs (hMSCs) and is widely utilized in osteogenic protocols. The aim of this study was to determine the effect of BMP-2 and dexamethasone on early- and late-stage osteogenesis of autologous and induced pluripotent stem cell (iPS)-derived cMSCs. Two preparations of marrow-derived cMSCs were selected to represent exceptionally or marginally osteogenic autologous cMSCs. iPS-derived cMSCs were generated from canine fibroblasts. All preparations were evaluated using alkaline phosphatase (ALP) activity, Alizarin Red staining of osteogenic monolayers, and quantitative polymerase chain reaction. Data were reported as mean ± standard deviation and compared using one- or two-way analysis of variance and Tukey or Sidak post hoc tests. Significance was established at P < 0.05. In early-stage assays, dexamethasone decreased ALP activity for all cMSCs in the presence of BMP-2. In late-stage assays, inclusion of dexamethasone and BMP-2 at Day 1 of culture produced robust monolayer mineralization for autologous cMSCs. Delivering 100 nM dexamethasone at Day 1 improved mineralization and reduced the BMP-2 concentrations required to achieve mineralization of the marginal cMSCs. For iPS-cMSCs, dexamethasone was inhibitory to both ALP activity and monolayer mineralization. There was increased expression of osteocalcin and osterix with BMP-2 in autologous cMSCs but a more modest expression occurred in iPS cMSCs. While autologous and iPS-derived cMSCs respond similarly in early-stage osteogenic assays, they exhibit unique responses to dexamethasone and BMP-2 in late-stage mineralization assays. This study demonstrates that dexamethasone and BMP-2 can be titrated in a time- and concentration-dependent manner to enhance osteogenesis of autologous cMSC preparations. These results will prove useful for investigators performing translational studies with cMSCs while providing insight into iPS-derived cMSC osteogenesis.

Endogenous Glucocorticoid Metabolism in Bone: Friend or Foe

The role of tissue specific metabolism of endogenous glucocorticoids (GCs) in the pathogenesis of human disease has been a field of intense interest over the last 20 years, fuelling clinical trials of metabolism inhibitors in the treatment of an array of metabolic diseases. Localised pre-receptor metabolism of endogenous and therapeutic GCs by the 11β-hydroxysteroid dehydrogenase (11β-HSD) enzymes (which interconvert endogenous GCs between their inactive and active forms) are increasingly recognised as being critical in mediating both their positive and negative actions on bone homeostasis. In this review we explore the roles of endogenous and therapeutic GC metabolism by the 11β-HSD enzymes in the context of bone metabolism and bone cell function, and consider future strategies aimed at modulating this system in order to manage and treat various bone diseases.

High bone marrow fat in patients with Cushing's syndrome and vertebral fractures

PURPOSE

The evaluation of skeletal fragility in Cushing's syndrome (CS) is a clinical challenge, since dual-energy X-ray absorptiometry (DXA) does not capture abnormalities in bone microstructure induced by glucocorticoid excess. Hypercortisolism was shown to increase bone marrow adiposity, but it is still unknown whether high bone marrow fat (BMF) as measured by vertebral magnetic resonance spectroscopy may predict fracture risk in this clinical setting. In this cross-sectional study, we evaluated the association between BMF and vertebral fractures (VFs) in patients with CS.

METHODS

Twenty patients (5 M, age 44 ± 13 years) with active CS were evaluated for morphometric VFs, lumbar spine BMF, and bone mineral density (BMD). Fifteen healthy volunteers (4 M, age 43 ± 12 years) acted as control group for BMF evaluation.

RESULTS

BMF was significantly higher in CS patients vs. controls (52.0% vs. 27.0%, p < 0.01), and was directly correlated with patients' age (p = 0.03), 24-hours urine-free cortisol (p = 0.03), midnight serum cortisol (p = 0.02), and serum CTX (p = 0.01). Patients with VFs (13 cases) showed significantly higher BMF vs. patients without VFs (65.0% vs. 24.0%, p = 0.03). Fractured patients with either normal BMD or osteopenia showed comparable BMF to fractured patients with either osteoporosis or low BMD for age (p = 0.71). When the analysis was restricted to patients with normal BMD or osteopenia, VFs were still significantly associated with higher BMF (p = 0.05).

CONCLUSIONS

This study provides a first evidence that vertebral adiposity may be a marker of hypercortisolism-induced skeletal fragility and measurement of spine BMF could have a role in the diagnostic work-up for the assessment of fracture risk in CS.

Adrenocortical incidentalomas and bone: from molecular insights to clinical perspectives

Adrenal incidentalomas constitute a common clinical problem with an overall prevalence of around 2-3%, but are more common with advancing age being present in 10% of those aged 70 years. The majority of these lesions are benign adrenocortical adenomas (80%), characterized in 10-40% of the cases by autonomous cortisol hypersecretion, and in 1-10% by aldosterone hypersecretion. Several observational studies have shown that autonomous cortisol and aldosterone hypersecretion are more prevalent than expected in patients with osteopenia and osteoporosis: these patients have accelerated bone loss and an increased incidence of vertebral fractures. In contrast to glucocorticoid action, the effects of aldosterone on bone are less well understood. Recent data, demonstrating a concomitant co-secretion of glucocorticoid metabolites in patients with primary aldosteronism, could explain some of the metabolic abnormalities seen in patients with aldosterone hypersecretion. In clinical practice, patients with unexplained osteoporosis, particularly when associated with other features such as impaired glucose tolerance or hypertension, should be investigated for the possible presence of autonomous cortisol or aldosterone secretion due to an adrenal adenoma. Randomized intervention studies are needed, however, to investigate the optimum interventions for osteoporosis and other co-morbidities in these patients.

Pituitary Diseases and Bone

Neuroendocrinology of bone is a new area of research based on the evidence that pituitary hormones may directly modulate bone remodeling and metabolism. Skeletal fragility associated with high risk of fractures is a common complication of several pituitary diseases such as hypopituitarism, Cushing disease, acromegaly, and hyperprolactinemia. As in other forms of secondary osteoporosis, pituitary diseases generally affect bone quality more than bone quantity, and fractures may occur even in the presence of normal or low-normal bone mineral density as measured by dual-energy X-ray absorptiometry, making difficult the prediction of fractures in these clinical settings. Treatment of pituitary hormone excess and deficiency generally improves skeletal health, although some patients remain at high risk of fractures, and treatment with bone-active drugs may become mandatory. The aim of this review is to discuss the physiological, pathophysiological, and clinical insights of bone involvement in pituitary diseases.

Prenatal nicotine exposure retards osteoclastogenesis and endochondral ossification in fetal long bones in rats

This study investigated the mechanisms underlying the retarded development of long bone in fetus by prenatal nicotine exposure (PNE) which had been demonstrated by our previous work. Nicotine (2.0 mg/kg.d) or saline was injected subcutaneously into pregnant rats every morning from gestational day (GD) 9 to 20. Fetal femurs or tibias were harvested for analysis on GD 20. We found massive accumulation of hypertrophic chondrocytes and a delayed formation of primary ossification center (POC) in the fetal femur or tibia of rat fetus after PNE, which was accompanied by a decreased amount of osteoclasts in the POC and up-regulated expression of osteoprotegerin (OPG) but by no obvious change in the expression of receptor activator of NF-κB ligand (RANKL). In primary osteoblastic cells, both nicotine (0, 162, 1620, 16,200 ng/ml) and corticosterone (0, 50, 250, 1250 nM) promoted the mRNA expression of OPG but concentration-dependently suppressed that of RANKL. Furthermore, blocking α4β2-nicotinic acetylcholine receptor (α4β2-nAChR) or glucocorticoid receptor rescued the above effects of nicotine and corticosterone, respectively. In conclusion, retarded osteoclastogenesis may contribute to delayed endochondral ossification in long bone in fetal rats with PNE. The adverse effects of PNE may be mediated via the direct effect of nicotine and indirect effect of maternal corticosterone on osteoblastic cells.

Glucocorticoids suppress Wnt16 expression in osteoblasts in vitro and in vivo

Glucocorticoid-induced osteoporosis is a frequent complication of systemic glucocorticoid (GC) therapy and mainly characterized by suppressed osteoblast activity. Wnt16 derived from osteogenic cells is a key determinant of bone mass. Here, we assessed whether GC suppress bone formation via inhibiting Wnt16 expression. GC treatment with dexamethasone (DEX) decreased Wnt16 mRNA levels in murine bone marrow stromal cells (mBMSCs) time- and dose-dependently. Similarly, Wnt16 expression was also suppressed after DEX treatment in calvarial organ cultures. Consistently, mice receiving GC-containing slow-release prednisolone pellets showed lower skeletal Wnt16 mRNA levels and bone mineral density than placebo-treated mice. The suppression of Wnt16 by GCs was GC-receptor-dependent as co-treatment of mBMSCs with DEX and the GR antagonist RU-486 abrogated the GC-mediated suppression of Wnt16. Likewise, DEX failed to suppress Wnt16 expression in GR knockout-mBMSCs. In addition, Wnt16 mRNA levels were unaltered in bone tissue of GC-treated GR dimerization-defective GR^dim mice, suggesting that GCs suppress Wnt16 via direct DNA-binding mechanisms. Consistently, DEX treatment reduced Wnt16 promoter activity in MC3T3-E1 cells. Finally, recombinant Wnt16 restored DEX-induced suppression of bone formation in mouse calvaria. Thus, this study identifies Wnt16 as a novel target of GC action in GC-induced suppression of bone formation.

Stress, Glucocorticoids and Bone: A Review From Mammals and Fish

Glucocorticoids (GCs) are the final effector products of a neuroendocrine HPA/HPI axis governing energy balance and stress response in vertebrates. From a physiological point of view, basal GC levels are essential for intermediary metabolism and participate in the development and homeostasis of a wide range of body tissues, including the skeleton. Numerous mammalian studies have demonstrated that GC hormones exert a positive role during bone modeling and remodeling as they promote osteoblastogenesis to maintain the bone architecture. Although the pharmacological effect of the so-called stress hormones has been widely reported, the role of endogenous GCs on bone mineral metabolism as result of the endocrine stress response has been largely overlooked across vertebrates. In addition, stress responses are variable depending on the stressor (e.g., starvation, predation, and environmental change), life cycle events (e.g., migration and aging), and differ among vertebrate lineages, which react differently according to their biological, social and cognitive complexity (e.g., mineral demands, physical, and psychological stress). This review intends to summarize the endogenous GCs action on bone metabolism of mammals and fish under a variety of challenging circumstances. Particular emphasis will be given to the regulatory loop between GCs and the parathyroid hormone (PTH) family peptides, and other key regulators of mineral homeostasis and bone remodeling in vertebrates.

Glucocorticoids promote a glioma stem cell-like phenotype and resistance to chemotherapy in human glioblastoma primary cells: Biological and prognostic significance

Glioma stem cells (GSCs) are glioblastoma (GBM) cells that are resistant to therapy and can give rise to recurrent tumors. The identification of patient-related factors that support GSCs is thus necessary to design effective therapies for GBM patients. Glucocorticoids (GCs) are used to treat GBM-associated edema. However, glucocorticoids participate in the physiological response to psychosocial stress, which has been linked to poor cancer prognosis. This raises concern that glucocorticoids affect the tumor and GSCs. Here, we treated primary human GBM cells with dexamethasone and evaluated GC-driven changes in cell morphology, proliferation, migration, gene expression, secretory activity and growth as neurospheres. Dexamethasone treatment of GBM cells appeared to promote the development of a GSC-like phenotype and conferred resistance to physiological stress and chemotherapy. We also analyzed a potential correlation between GC treatment and tumor recurrence after surgical excision in a population-based consecutive cohort of 48 GBM patients, adjusted for differences in known prognostic factors concerning baseline and treatment characteristics. In this cohort, we found a negative correlation between GC intake and progression-free survival, regardless of the MGMT methylation status. In conclusion, our findings raise concern that treatment of GBM with GCs may compromise the efficacy of chemotherapy and may support a GSC population, which could contribute to tumor recurrence and the poor prognosis of the disease.

The effect of clinically relevant doses of immunosuppressive drugs on human mesenchymal stem cells

Immunosuppressive drugs are used to suppress graft rejection after transplantation and for the treatment of various diseases. The main limitations of their use in clinical settings are severe side effects, therefore alternative approaches are desirable. In this respect, mesenchymal stem cells (MSCs) possess a regenerative and immunomodulatory capacity that has generated considerable interest for their use in cell-based therapy. Currently, MSCs are tested in many clinical trials, including the treatment of diseases which require simultaneous immunosuppressive treatment. Since the molecular targets of immunosuppressive drugs are also present in MSCs, we investigated whether immunosuppressive drugs interact with the activity of MSCs. Human MSCs isolated from the bone marrow (BM) or adipose tissue (AT) were cultured in the presence of clinical doses of five widely used immunosuppressive drugs (cyclosporine A, mycophenolate mofetil, rapamycin, prednisone and dexamethasone), and the influence of these drugs on several factors related to the immunosuppressive properties of MSCs, including the expression of immunomodulatory enzymes, various growth factors, cytokines, chemokines, adhesion molecules and proapoptotic ligands, was assessed. Glucocorticoids, especially dexamethasone, showed the most prominent effects on both types of MSCs and suppressed the expression of the majority of the factors that were tested. A significant increase of hepatocyte growth factor production in AT-MSCs and of indoleamine 2,3-dioxygenase expression in both types of MSCs were the only exceptions. In conclusion, clinically relevant doses of inhibitors of calcineurin, mTOR and IMPDH and glucocorticoids interfere with MSC functions, but do not restrain their immunosuppressive properties. These findings should be taken into account before preparing immunosuppressive strategies combining the use of immunosuppressive drugs and MSCs.

Proteasome inhibitor bortezomib is a novel therapeutic agent for focal radiation-induced osteoporosis

Bone atrophy and its related fragility fractures are frequent, late side effects of radiotherapy in cancer survivors and have a detrimental impact on their quality of life. In another study, we showed that parathyroid hormone 1-34 and anti-sclerostin antibody attenuates radiation-induced bone damage by accelerating DNA repair in osteoblasts. DNA damage responses are partially regulated by the ubiquitin proteasome pathway. In the current study, we examined whether proteasome inhibitors have similar bone-protective effects against radiation damage. MG132 treatment greatly reduced radiation-induced apoptosis in cultured osteoblastic cells. This survival effect was owing to accelerated DNA repair as revealed by γH2AX foci and comet assays and to the up-regulation of Ku70 and DNA-dependent protein kinase, catalytic subunit, essential DNA repair proteins in the nonhomologous end-joining pathway. Administration of bortezomib (Bzb) reversed the loss of trabecular bone structure and strength in mice at 4 wk after focal radiation. Histomorphometry revealed that Bzb significantly increased the number of osteoblasts and activity in the irradiated area and suppressed the number and activity of osteoclasts, regardless of irradiation. Two weeks of Bzb treatment accelerated DNA repair in bone-lining osteoblasts and thus promoted their survival. Meanwhile, it also inhibited bone marrow adiposity. Taken together, we demonstrate a novel role of proteasome inhibitors in treating radiation-induced osteoporosis.-Chandra, A., Wang, L., Young, T., Zhong, L., Tseng, W.-J., Levine, M. A., Cengel, K., Liu, X. S., Zhang, Y., Pignolo, R. J., Qin, L. Proteasome inhibitor bortezomib is a novel therapeutic agent for focal radiation-induced osteoporosis.

Glucocorticoid receptor binding to chromatin is selectively controlled by the coregulator Hic-5 and chromatin remodeling enzymes

The steroid hormone-activated glucocorticoid receptor (GR) regulates cellular stress pathways by binding to genomic regulatory elements of target genes and recruiting coregulator proteins to remodel chromatin and regulate transcription complex assembly. The coregulator hydrogen peroxide-inducible clone 5 (Hic-5) is required for glucocorticoid (GC) regulation of some genes but not others and blocks the regulation of a third gene set by inhibiting GR binding. How Hic-5 exerts these gene-specific effects and specifically how it blocks GR binding to some genes but not others is unclear. Here we show that site-specific blocking of GR binding is due to gene-specific requirements for ATP-dependent chromatin remodeling enzymes. By depletion of 11 different chromatin remodelers, we found that ATPases chromodomain helicase DNA-binding protein 9 (CHD9) and Brahma homologue (BRM, a product of the SMARCA2 gene) are required for GC-regulated expression of the blocked genes but not for other GC-regulated genes. Furthermore, CHD9 and BRM were required for GR occupancy and chromatin remodeling at GR-binding regions associated with blocked genes but not at GR-binding regions associated with other GC-regulated genes. Hic-5 selectively inhibits GR interaction with CHD9 and BRM, thereby blocking chromatin remodeling and robust GR binding at GR-binding sites associated with blocked genes. Thus, Hic-5 regulates GR binding site selection by a novel mechanism, exploiting gene-specific requirements for chromatin remodeling enzymes to selectively influence DNA occupancy and gene regulation by a transcription factor.

Tetramethylpyrazine Protects Against Glucocorticoid-Induced Apoptosis by Promoting Autophagy in Mesenchymal Stem Cells and Improves Bone Mass in Glucocorticoid-Induced Osteoporosis Rats

Glucocorticoid-induced osteoporosis (GIOP) is a widespread clinical complication due to the common use of glucocorticoids. Excess glucocorticoids induce apoptosis of bone marrow-derived mesenchymal stem cells (BMSCs), which have been shown to play an increasingly important role in the pathogenesis and therapy of osteoporosis. Tetramethylpyrazine (TMP), an extract from one of the most recognized herbs in traditional Chinese medicine (Chuanxiong), has been reported to have antiapoptotic properties. In this study, we tested whether TMP protects rat BMSCs following exposure to glucocorticoids in vitro and in vivo. We treated BMSCs with different concentrations of TMP (50, 100, or 200 μM) and exposed them to 10^-6 M dexamethasone (Dex) for 48 h in vitro. Our data showed that TMP inhibited Dex-induced cytotoxicity and protected BMSCs from apoptosis. Interestingly, further results demonstrated that TMP prevented apoptosis in BMSCs by promoting autophagy in an AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) pathway-dependent manner. In addition, calcein fluorescence double labeling and microcomputed tomography scanning indicated that 12 weeks of TMP administration augmented bone formation and protected trabecular bone mass in GIOP rats. We also discovered that first-passage BMSCs isolated from the TMP treatment group had a lower rate of apoptosis and a higher light chain 3 (LC3)-II/LC3-I ratio than the GIOP group. Our findings demonstrate for the first time that TMP can protect BMSCs from exposure to excess glucocorticoids by promoting autophagy through AMPK/mTOR pathway and might be an effective agent for the prevention and treatment of GIOP.

Molecular Actions of Glucocorticoids in Cartilage and Bone During Health, Disease, and Steroid Therapy

Cartilage and bone are severely affected by glucocorticoids (GCs), steroid hormones that are frequently used to treat inflammatory diseases. Major complications associated with long-term steroid therapy include impairment of cartilaginous bone growth and GC-induced osteoporosis. Particularly in arthritis, GC application can increase joint and bone damage. Contrarily, endogenous GC release supports cartilage and bone integrity. In the last decade, substantial progress in the understanding of the molecular mechanisms of GC action has been gained through genome-wide binding studies of the GC receptor. These genomic approaches have revolutionized our understanding of gene regulation by ligand-induced transcription factors in general. Furthermore, specific inactivation of GC signaling and the GC receptor in bone and cartilage cells of rodent models has enabled the cell-specific effects of GCs in normal tissue homeostasis, inflammatory bone diseases, and GC-induced osteoporosis to be dissected. In this review, we summarize the current view of GC action in cartilage and bone. We further discuss future research directions in the context of new concepts for optimized steroid therapies with less detrimental effects on bone.

Mitogen-inducible gene-6 partly mediates the inhibitory effects of prenatal dexamethasone exposure on endochondral ossification in long bones of fetal rats

BACKGROUND AND PURPOSE

Prenatal exposure to dexamethasone slows down fetal linear growth and bone mineralization but the regulatory mechanism remains unknown. Here we assessed how dexamethasone regulates bone development in the fetus.

EXPERIMENTAL APPROACH

Dexamethasone (1 mg·kg(-1) ·day(-1) ) was injected subcutaneously every morning in pregnant rats from gestational day (GD)9 to GD20. Fetal femurs and tibias were harvested at GD20 for histological and gene expression analysis. Femurs of 12-week-old female offspring were harvested for microCT (μCT) measurement. Primary chondrocytes were treated with dexamethasone (10, 50, 250 and 1000 nM).

KEY RESULTS

Prenatal dexamethasone exposure resulted in accumulation of hypertrophic chondrocytes and delayed formation of the primary ossification centre in fetal long bone. The retardation was accompanied by reduced maturation of hypertrophic chondrocytes, decreased osteoclast number and down-regulated expression of osteocalcin and bone sialoprotein in long bone. In addition, the mitogen-inducible gene-6 (Mig6) and osteoprotegerin (OPG) expression were stimulated, and the receptor activator of NF-κB ligand (RANKL) expression was repressed. Moreover, dexamethasone activated OPG and repressed RANKL expression in both primary chondrocytes and primary osteoblasts, and the knockdown of Mig6 abolished the effect of dexamethasone on OPG expression. Further, μCT measurement showed loss of bone mass in femur of 12-week-old offspring with prenatal dexamethasone exposure.

CONCLUSIONS AND IMPLICATIONS

Prenatal dexamethasone exposure delays endochondral ossification by suppressing chondrocyte maturation and osteoclast differentiation, which may be partly mediated by Mig6 activation in bone. Bone development retardation in the fetus may be associated with reduced bone mass in later life.

Glucocorticoid-Induced Osteoporosis

Osteoporosis is among the most devastating side effects of glucocorticoid (GC) therapy for the management of inflammatory and auto-immune diseases. Evidence from both humans and mice indicate deleterious skeletal effects within weeks of pharmacological GC administration, both related and unrelated to a decrease in bone mineral density (BMD). Osteoclast numbers and bone resorption are also rapidly increased, and together with osteoblast inactivation and decreased bone formation, these changes lead the fastest loss in BMD during the initial disease phase. Bone resorption then decreases to sub-physiological levels, but persistent and severe inhibition of bone formation leads to further bone loss and progressively increased fracture risk, up to an order of magnitude higher than that observed in untreated individuals. Bone forming osteoblasts are thus considered the main culprits in GC-induced osteoporosis (GIO). Accordingly, we focus this review primarily on deleterious effects on osteoblasts: inhibition of cell replication and function and acceleration of apoptosis. Mediating these adverse effects, GCs target pivotal regulatory mechanisms that govern osteoblast growth, differentiation and survival. Specifically, GCs inhibit growth factor pathways, including Insulin Growth Factors, Growth Hormone, Hepatocyte Growth/Scatter Factor and IL6-type cytokines. They also inhibit downstream kinases, including PI3-kinase and the MAP kinase ERK, the latter attributable in part to direct transcriptional stimulation of MAP kinase phosphatase 1. Most importantly, however, GCs inhibit the Wnt signaling pathway, which plays a pivotal role in osteoblast replication, function and survival. They transcriptionally stimulate expression of Wnt inhibitors of both the Dkk and Sfrp families, and they induce reactive oxygen species (ROS), which result in loss of ß-catenin to ROS-activated FoxO transcription factors. Identification of dissociated GCs, which would suppress the immune system without causing osteoporosis, is proving more challenging than initially thought, and GIO is currently managed by co-treatment with bisphosphonates or PTH. These drugs, however, are not ideally suited for GIO. Future therapeutic approaches may aim at GC targets such as those mentioned above, or newly identified targets including the Notch pathway, the AP-1/Il11 axis and the osteoblast master regulator RUNX2.

The effect of laser therapy on the expression of osteocalcin and osteopontin after tooth extraction in rats treated with zoledronate and dexamethasone

PURPOSE

Laser therapy has been used for the prevention and management of medication-related ostenecrosis of the jaw (MRONJ). The aim of this paper was to investigate the action of laser therapy on extraction socket healing in rats in conditions at risk for MRONJ, evaluating the expression of markers of bone metabolism.

METHODS

Thirty male Sprague-Dawley rats were divided in four groups: control group (C, n = 5), laser group (L, n = 5), treatment group (T, n = 10), and treatment plus laser group (T + L, n = 10). Rats of group T and T + L received zoledronate 0.1 mg/kg and dexamethasone 1 mg/kg every 2 days for 10 weeks. Rats of group C and L were infused with vehicle. After 9 weeks, the left maxillary molars were extracted in all rats. Rats of groups L and T + L received laser therapy (Nd:YAG, 1064 nm, 1.25 W, 15 Hz, 5 min, 14.37 J/cm(2)) in the socket area at days 0, 2, 4, and 6 after surgery. Western blot analysis was performed to evaluate the alveolar expression of osteopontin (OPN) and osteocalcin (OCN) 8 days after extraction.

RESULTS

Rats of groups L and T + L showed a significant higher expression of OCN compared to rats of groups C and T (+348 and +400 %, respectively; P = 0.013 and P = 0.002, respectively). The expression of OPN did not show significant differences among the different groups.

CONCLUSIONS

Our findings suggest that laser irradiation after tooth extraction can promote osteoblast differentiation, as demonstrated by the higher expression of OCN. Thus, laser irradiation could be considered a way to improve socket healing in conditions at risk for MRONJ development.

PTH1-34 blocks radiation-induced osteoblast apoptosis by enhancing DNA repair through canonical Wnt pathway

Focal radiotherapy for cancer patients has detrimental effects on bones within the radiation field and the primary clinical signs of bone damage include the loss of functional osteoblasts. We reported previously that daily injection of parathyroid hormone (PTH, 1-34) alleviates radiation-induced osteopenia in a preclinical radiotherapy model by improving osteoblast survival. To elucidate the molecular mechanisms, we irradiated osteoblastic UMR 106-01 cells and calvarial organ culture and demonstrated an anti-apoptosis effect of PTH1-34 on these cultures. Inhibitor assay indicated that PTH exerts its radioprotective action mainly through protein kinase A/β-catenin pathway. γ-H2AX foci staining and comet assay revealed that PTH efficiently promotes the repair of DNA double strand breaks (DSBs) in irradiated osteoblasts via activating the β-catenin pathway. Interestingly, Wnt3a alone also blocked cell death and accelerated DNA repair in primary osteoprogenitors, osteoblastic and osteocytic cells after radiation through the canonical signaling. Further investigations revealed that both Wnt3a and PTH increase the amount of Ku70, a core protein for initiating the assembly of DSB repair machinery, in osteoblasts after radiation. Moreover, down-regulation of Ku70 by siRNA abrogated the prosurvival effect of PTH and Wnt3a on irradiated osteoblasts. In summary, our results identify a novel role of PTH and canonical Wnt signaling in regulating DSB repair machinery and apoptosis in osteoblasts and shed light on using PTH1-34 or Wnt agonist as possible therapy for radiation-induced osteoporosis.

A facile in vitro model to study rapid mineralization in bone tissues

Background

Mineralization in bone tissue involves stepwise cell-cell and cell-ECM interaction. Regulation of osteoblast culture microenvironments can tailor osteoblast proliferation and mineralization rate, and the quality and/or quantity of the final calcified tissue. An in vitro model to investigate the influencing factors is highly required.

Methods

We developed a facile in vitro model in which an osteoblast cell line and aggregate culture (through the modification of culture well surfaces) were used to mimic intramembranous bone mineralization. The effect of culture environments including culture duration (up to 72 hours for rapid mineralization study) and aggregates size (monolayer culture as control) on mineralization rate and mineral quantity/quality were examined by osteogenic gene expression (PCR) and mineral markers (histological staining, SEM-EDX and micro-CT).

Results

Two size aggregates (on average, large aggregates were 745 μm and small 79 μm) were obtained by the facile technique with high yield. Cells in aggregate culture generated visible and quantifiable mineralized matrix within 24 hours, whereas cells in monolayer failed to do so by 72 hours. The gene expression of important ECM molecules for bone formation including collagen type I, alkaline phosphatase, osteopontin and osteocalcin, varied temporally, differed between monolayer and aggregate cultures, and depended on aggregate size. Monolayer specimens stayed in a proliferation phase for the first 24 hours, and remained in matrix synthesis up to 72 hours; whereas the small aggregates were in the maturation phase for the first 24 and 48 hour cultures and then jumped to a mineralization phase at 72 hours. Large aggregates were in a mineralization phase at all these three time points and produced 36% larger bone nodules with a higher calcium content than those in the small aggregates after just 72 hours in culture.

Conclusions

This study confirms that aggregate culture is sufficient to induce rapid mineralization and that aggregate size determines the mineralization rate. Mineral content depended on aggregate size and culture duration. Thus, our culture system may provide a good model to study regulation factors at different development phases of the osteoblastic lineage.

Electronic supplementary material

The online version of this article (doi:10.1186/1475-925X-13-136) contains supplementary material, which is available to authorized users.

Osteogenic differentiation of bone mesenchymal stem cells regulated by osteoblasts under EMF exposure in a co-culture system

This study examined the osteogenic effect of electromagnetic fields (EMF) under the simulated in vivo conditions. Rat bone marrow mesenchymal stem cells (BMSCs) and rat osteoblasts were co-cultured and exposed to 50 Hz, 1.0 mT EMF for different terms. Unexposed single-cultured BMSCs and osteoblasts were set as controls. Cell proliferation features of single-cultured BMSCs and osteoblasts were studied by using a cell counting kit (CCK-8). For the co-culture system, cells in each group were randomly chosen for alkaline phosphatase (ALP) staining on the day 7. When EMF exposure lasted for 14 days, dishes in each group were randomly chosen for total RNA extraction and von Kossa staining. The mRNA expression of osteogenic markers was detected by using real-time PCR. Our study showed that short-term EMF exposure (2 h/day) could obviously promote proliferation of BMSCs and osteoblasts, while long-term EMF (8 h/day) could promote osteogenic differentiation significantly under co-cultured conditions. Under EMF exposure, osteogenesis-related mRNA expression changed obviously in co-cultured and single-cultured cells. It was noteworthy that most osteogenic indices in osteoblasts were increased markedly after co-culture except Bmp2, which was increased gradually when cells were exposed to EMF. Compared to other indices, the expression of Bmp2 in BMSCs was increased sharply in both single-cultured and co-cultured groups when they were exposed to EMF. The mRNA expression of Bmp2 in BMSCs was approximately four times higher in 8-h EMF group than that in the unexposed group. Our results suggest that Bmp2-mediated cellular interaction induced by EMF exposure might play an important role in the osteogenic differentiation of BMSCs.

Determinants of bone marrow adiposity: the modulation of peroxisome proliferator-activated receptor-γ2 activity as a central mechanism

Although the presence of adipocytes in the bone marrow is a normal physiological phenomenon, the role of these cells in bone homeostasis and during pathological states has not yet been fully delineated. As osteoblasts and adipocytes originate from a common progenitor, with an inverse relationship existing between osteoblastogenesis and adipogenesis, bone marrow adiposity often negatively correlates with osteoblast number and bone mineral density. Bone adiposity can be affected by several physiological and pathophysiological factors, with abnormal, elevated marrow fat resulting in a pathological state. This review focuses on the regulation of bone adiposity by physiological factors, including aging, mechanical loading and growth factor expression, as well as the pathophysiological factors, including diseases such as anorexia nervosa and dyslipidemia, and pharmacological agents such as thiazolidinediones and statins. Although these factors regulate bone marrow adiposity via a plethora of different intracellular signaling pathways, these diverse pathways often converge on the modulation of the expression and/or activity of the pro-adipogenic transcription factor peroxisome proliferator-activated receptor (PPAR)-γ2, suggesting that any factor that affects PPAR-γ2 may have an impact on the fat content of bone.

The effects of dexamethasone on the apoptosis and osteogenic differentiation of human periodontal ligament cells

PURPOSE

The purpose of the current study was to examine the effect of dexamethasone (Dex) at various concentrations on the apoptosis and mineralization of human periodontal ligament (hPDL) cells.

METHODS

hPDL cells were obtained from the mid-third of premolars extracted for orthodontic reasons, and a primary culture of hPDL cells was prepared using an explant technique. Groups of cells were divided according to the concentration of Dex (0, 1, 10, 100, and 1,000 nM). A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay was performed for evaluation of cellular viability, and alkaline phosphatase activity was examined for osteogenic differentiation of hPDL cells. Alizarin Red S staining was performed for observation of mineralization, and real-time polymerase chain reaction was performed for the evaluation of related genes.

RESULTS

Increasing the Dex concentration was found to reduce cellular viability, with an increase in alkaline phosphatase activity and mineralization. Within the range of Dex concentrations tested in this study, 100 nM of Dex was found to promote the most vigorous differentiation and mineralization of hPDL cells. Dex-induced osteogenic differentiation and mineralization was accompanied by an increase in the level of osteogenic and apoptosis-related genes and a reduction in the level of antiapoptotic genes. The decrease in hPDL cellular viability by glucocorticoid may be explained in part by the increased prevalence of cell apoptosis, as demonstrated by BAX expression and decreased expression of the antiapoptotic gene, Bcl-2.

CONCLUSIONS

An increase in hPDL cell differentiation rather than cellular viability at an early stage is likely to be a key factor in glucocorticoid induced mineralization. In addition, apoptosis might play an important role in Dex-induced tissue regeneration; however, further study is needed for investigation of the precise mechanism.

Up-regulation of Axin2 by dexamethasone promotes adipocyte differentiation in ROB-C26 mesenchymal progenitor cells

Dexamethasone (Dex) regulates osteoblastic and adipocytic differentiation in mesenchymal progenitor cells through regulation of Wnt/β-catenin signaling. To elucidate the regulatory mechanisms underlying the effects of Dex, we examine the expression of Axin2, which is an intracellular inhibitor of Wnt/β-catenin signaling, in ROB-C26 clonal mesenchymal progenitor cells (C26). We observed the induction of Axin2 mRNA in C26 cells in response to Dex treatment. Treatment with a glucocorticoid receptor (GR) antagonist, mifepristone, showed that Dex-induced up-regulation of Axin2 is mediated by the GR. In the absence of Dex, gene silencing by using Axin2-targeted short hairpin RNA increased the number of alkaline phosphatase (ALP)-positive and nuclear β-catenin-positive cells and ALP activity. In the presence of Dex, Axin2 knockdown resulted in an increased number of ALP-positive and nuclear β-catenin-positive cells. Furthermore, Axin2 knockdown in Dex-treated cells suppressed adipocyte differentiation (as determined by reduced Oil Red O staining), reduced the number of PPARγ-positive and aP2-positive cells and decreased the mRNA expression of PPARγ2 and aP2. These results suggest that Axin2 plays a key role in adipocyte and osteoblastic differentiation by controlling β-catenin expression.

The interrelationship between bone and fat: from cellular see-saw to endocrine reciprocity

The number of mature osteoblasts and marrow adipocytes in bone is influenced by the differentiation of the common mesenchymal progenitor cell towards one phenotype and away from the other. Consequently, factors which promote adipogenesis not only lead to fatty marrow but also inhibit osteoblastogenesis, resulting in decreased osteoblast numbers, diminished bone formation and, potentially, inadequate bone mass and osteoporosis. In addition to osteoblast and bone adipocyte numbers being influenced by this skewing of progenitor cell differentiation towards one phenotype, mature osteoblasts and adipocytes secrete factors which may evoke changes in the cell fate and function of each other. This review examines the endogenous factors, such as PPAR-γ2, Wnt, IGF-1, GH, FGF-2, oestrogen, the GP130 signalling cytokines, vitamin D and glucocorticoids, which regulate the selection between osteoblastogenesis and adipogenesis and the interrelationship between fat and bone. The role of adipokines on bone, such as adiponectin and leptin, as well as adipose-derived oestrogen, is reviewed and the role of bone as an energy regulating endocrine organ is discussed.

Endogenous Glucocorticoids and Bone

While the adverse effects of glucocorticoids on bone are well described, positive effects of glucocorticoids on the differentiation of osteoblasts are also observed. These paradoxical effects of glucocorticoids are dose dependent. At both physiologicaland supraphysiological levels of glucocorticoids, osteoblasts and osteocytes are the major glucocorticoid target cells. However, the response of the osteoblasts to each of these is quite distinct. At physiology levels, glucocorticoids direct mesenchymal progenitor cells to differentiate towards osteoblasts and thus increase bone formation in a positive way. In contrast with ageing, the excess production of glucocorticoids, at both systemic and intracellular levels, appear to impact on osteoblast and osteocytes in a negative way in a similar fashion to that seen with therapeutic glucocorticoids. This review will focus on therole of glucocorticoids in normal bone physiology, with particular emphasis on the mechanism by which endogenous glucocorticoids impact on bone and its constituent cells.

Epidermal growth factor receptor (EGFR) signaling promotes proliferation and survival in osteoprogenitors by increasing early growth response 2 (EGR2) expression

Maintaining bone architecture requires continuous generation of osteoblasts from osteoprogenitor pools. Our previous study of mice with epidermal growth factor receptor (EGFR) specifically inactivated in osteoblast lineage cells revealed that EGFR stimulates bone formation by expanding the population of mesenchymal progenitors. EGFR ligands are potent regulators for the osteoprogenitor pool, but the underlying mechanisms are largely unknown. Here we demonstrate that activation of EGFR increases the number of osteoprogenitors by promoting cell proliferation and suppressing either serum depletion-induced or TNFα-induced apoptosis mainly through the MAPK/ERK pathway. Mouse calvarial organ culture revealed that EGF elevated the number of proliferative cells and decreased the number of apoptotic cells, which led to increased osteoblasts. Microarray analysis of MC3T3 cells, an osteoprogenitor cell line, revealed that EGFR signaling stimulates the expression of MCL1, an antiapoptotic protein, and a family of EGR transcription factors (EGR1, -2, and -3). The up-regulation of MCL1 and EGR2 by EGF was further confirmed in osteoprogenitors close to the calvarial bone surface. Overexpression of NAB2, a co-repressor for EGRs, attenuated the EGF-induced increase in osteoprogenitor number. Interestingly, knocking down the expression of EGR2, but not EGR1 or -3, resulted in a similar effect. Using inhibitor, adenovirus overexpression, and siRNA approaches, we demonstrate that EGFR signaling activates the MAPK/ERK pathway to stimulate the expression of EGR2, which in turn leads to cell growth and MCL1-mediated cell survival. Taken together, our data clearly demonstrate that EGFR-induced EGR2 expression is critical for osteoprogenitor maintenance and new bone formation.

Role of glucocorticoid in developmental programming: evidence from zebrafish

The vertebrate corticosteroid stress response is highly conserved and a key function is to restore homeostasis by mobilizing and reallocating energy stores. This process is primarily initiated by activation of the hypothalamus-pituitary-adrenal axis, leading to the release of corticosteroids into the circulation. In teleosts, cortisol is the primary corticosteroid that is released into the circulation in response to stress. This steroid activates corticosteroid receptors that are ligand-bound transcription factors, modulating downstream gene expression in target tissues. Recent research in zebrafish (Danio rerio) has identified novel roles for cortisol in early developmental processes, including organogenesis and mesoderm formation. As cortisol biosynthesis commences only around the time of hatch in teleosts, the early developmental events are orchestrated by cortisol that is maternally deposited prior to fertilization. This review will highlight the molecular events leading to the development of the corticosteroid stress axis, and the possible role of cortisol in the developmental programming of stress axis function. Use of zebrafish as a model may lead to significant insights into the conserved role of glucocorticoids during early development with potential implications in biomedical research, including fetal stress syndromes in humans.

EGFR signaling suppresses osteoblast differentiation and inhibits expression of master osteoblastic transcription factors Runx2 and Osterix

The epidermal growth factor receptor (EGFR) and its ligands regulate key processes of cell biology, such as proliferation, survival, differentiation, migration, and tumorigenesis. We previously showed that, EGFR signaling pathway is an important bone regulator and it primarily plays an anabolic role in bone metabolism. In this study, we demonstrated that EGF-like ligands strongly inhibited osteoblast differentiation and mineralization in several lines of osteoblastic cells. Real-time RT-PCR and promoter reporter assays revealed that EGF-like ligands suppressed the expression of both early and late bone marker genes at the transcriptional level in the differentiating osteoblasts via an EGFR-dependent manner. This inhibitory effect of EGFR signaling was not dependent on its mitogenic activity. Furthermore, we demonstrated that EGFR signaling reduced the expression of two major osteoblastic transcription factors Runx2 (type II) and Osterix in osteoblast differentiating cells. EGFR-induced decrease in Runx2 transcriptional activity was confirmed by Runx2 reporter and chromatin immunoprecipitation assays. EGFR signaling increased the protein amounts of transcription co-repressors HDAC4 and 6 and over-expression of HDAC4 decreased Runx2 amount in differentiating osteoblasts, implying that HDACs contribute to the down-regulation of Runx2 by EGFR. Moreover, activation of EGFR in undifferentiated osteoprogenitors attenuated the expression of early bone markers and Osterix and decreased Runx2 protein amounts. Together with our previous data, that EGFR stimulates osteoprogenitor proliferation and that blocking EGFR activity in osteoblast lineage cells results in fewer osteoprogenitors and an osteopenic phenotype, we conclude that EGFR signaling is important for maintaining osteoprogenitor population at an undifferentiated stage.

Dexamethasone inhibits bone resorption by indirectly inducing apoptosis of the bone-resorbing osteoclasts via the action of osteoblastic cells

Although glucocorticoids (GCs) are physiologically essentialfor bone metabolism, it is generally accepted that high dosesof GCs cause bone loss through a combination of decreased boneformation and increased bone resorption. However, the actionof GCs on mature osteoclasts remains contradictory. In thisstudy, we have examined the effect of GCs on osteoclasticbone-resorbing activity and osteoclast apoptosis, by using twodifferent cell types, rabbit unfractionated bone cells andhighly enriched mature osteoclasts (>95% of purity).Dexamethasone (Dex, 10(-10)-10(-7) M) inhibited resorption pit formation on a dentine slice by the unfractionated bone cells in a dose- and time-dependent manner.However, Dex had no effect on the bone-resorbing activity of the isolated mature osteoclasts. When the isolated osteoclastswere co-cultured with rabbit osteoblastic cells, the osteoclastic bone resorption decreased in response to Dex,dependent on the number of osteoblastic cells. Like the effecton the bone resorption, Dex induced osteoclast apoptosis in cultures of the unfractionated bone cells, whereas it did not promote the apoptosis of the isolated osteoclasts. An inhibitorof caspases, Z-Asp-CH2-DCB attenuated both the inhibitory effecton osteoclastic bone resorption and the stimulatory effect onthe osteoclast apoptosis. In addition, the osteoblastic cellswere required for the osteoclast apoptosis induced by Dex. These findings indicate that the main target cells of GCs arenon-osteoclastic cells such as osteoblasts and that GCsindirectly inhibit bone resorption by inducing apoptosis ofthe mature osteoclasts through the action of non-osteoclasticcells. This study expands our knowledge about the multifunctional roles of GCs in bone metabolism.

Short-term application of dexamethasone enhances bone morphogenetic protein-7-induced ectopic bone formation in vivo

BACKGROUND

Long-term administration of glucocorticoids may lead to bone loss and osteoporosis as reported in previous experimental and clinical studies. On the other hand, several in vitro studies have demonstrated that dexamethasone treatment induces proliferation and differentiation of human and murine osteoblast precursors. Thereby, a positive interaction of dexamethasone with the osteoinductive bone morphogenetic proteins (BMPs) is reported in vitro, but in vivo studies are still missing. Thus, the aim of this study was to determine whether short-term application of dexamethasone may improve BMP-7-induced bone formation in vivo.

METHODS

Ectopic bone formation was induced in control and dexamethasone-treated mice by application of BMP-7 into the hamstring muscles. After 20 days of treatment, each ectopic bone nodule was analyzed by contact radiography, microcomputed tomography, and histomorphometry. Furthermore, mice were subjected to histomorphometric analyses of their lumbar vertebrae and proximal tibiae to assess the systemic effect of short-term dexamethasone treatment on bone metabolism.

RESULTS

Dexamethasone application significantly increased the bone volume and osteoblast number of the ectopic bone nodules compared with untreated controls. Histomorphometric analyses of the lumbar vertebrae and proximal tibiae revealed no significant differences between the control and dexamethasone-treated mice.

CONCLUSIONS

This study demonstrates that BMP-7-induced ectopic bone formation is significantly enhanced by systemic short-term application of dexamethasone. These in vivo data confirm the results of previous in vitro studies and could be of interest for further studies with the intention to improve BMP-induced bone formation by short-term application of dexamethasone.

Glucocorticoid-induced osteoporosis - a disorder of mesenchymal stromal cells?

Glucocorticoids are a class of steroid hormones that are essential to life but cause serious harm in excess. The main clinical features of glucocorticoid excess are due to adverse effects on cells and tissues that arise from a common developmental precursor - the mesenchymal stromal cell (MSC; sometimes referred to as the mesenchymal stem cell). Interestingly glucocorticoids appear essential for the differentiation of cells and tissues that arise from MSCs. High levels of glucocorticoids are used in tissue engineering strategies to enhance the formation of tissues such as bone, cartilage, and muscle. This article discusses the paradox that glucocorticoids both enhance and impair MSC development and function. It will describe how endogenous glucocorticoids are likely to be important in these processes in vivo and will discuss the implications for therapies aimed at reducing the damage associated with the use of therapeutic glucocorticoids.

Ethanol alters the osteogenic differentiation of amniotic fluid-derived stem cells

BACKGROUND

Fetal alcohol spectrum disorder (FASD) is a set of developmental defects caused by prenatal alcohol exposure. Clinical manifestations of FASD are highly variable and include mental retardation and developmental defects of the heart, kidney, muscle, skeleton, and craniofacial structures. Specific effects of ethanol on fetal cells include induction of apoptosis as well as inhibition of proliferation, differentiation, and migration. This complex set of responses suggests that a bioinformatics approach could clarify some of the pathways involved in these responses.

METHODS

In this study, the responses of fetal stem cells derived from the amniotic fluid (AFSCs) to treatment with ethanol have been examined. Large-scale transcriptome analysis of ethanol-treated AFSCs indicates that genes involved in skeletal development and ossification are up-regulated in these cells. Therefore, the effect of ethanol on osteogenic differentiation of AFSCs was studied.

RESULTS

Exposure to ethanol during the first 48 hours of an osteogenic differentiation protocol increased in vitro calcium deposition by AFSCs and increased alkaline phosphatase activity. In contrast, ethanol treatment later in the differentiation protocol (day 8) had no significant effect on the activity of alkaline phosphatase.

CONCLUSIONS

These results suggest that transient exposure of AFSCs to ethanol during early differentiation enhances osteogenic differentiation of the cells.

Parathyroid hormone activation of matrix metalloproteinase-13 transcription requires the histone acetyltransferase activity of p300 and PCAF and p300-dependent acetylation of PCAF

Parathyroid hormone (PTH) regulates the transcription of many genes involved in bone remodeling in osteoblasts. One of these genes is matrix metalloproteinase-13 (MMP-13), which is involved in bone remodeling and early stages of endochondral bone formation. We have previously shown that Mmp-13 gene expression is highly induced by PTH treatment in osteoblastic UMR 106-01 cells, as well as primary osteoblasts. Here, we show that p300/CBP-associated factor (PCAF), in addition to p300 and Runx2, is required for PTH activation of Mmp-13 transcription. PCAF was increasingly recruited to the MMP-13 proximal promoter region after PTH treatment, and this was associated with an increase in RNA polymerase II recruitment and histone acetylation. In addition, PTH treatment increased the acetylation of PCAF, a process that required p300. Knockdown of PCAF, p300, or Runx2 by siRNA decreased Mmp-13 mRNA expression after PTH treatment in both UMR 106-01 cells and primary osteoblasts. We found that there is a mutual dependence between p300 and PCAF to be recruited to the Mmp-13 promoter after PTH treatment. In promoter-reporter assays, p300 and PCAF had an additive effect on PTH stimulation of MMP-13 promoter activity, and this required their histone acetyltransferase activity. Our findings demonstrate that PCAF acts downstream of PTH signaling as a transcriptional coactivator that is required for PTH stimulation of MMP-13 transcription. PCAF cooperates with p300 and Runx2 to mediate PTH activation of MMP-13 transcription.

Glucocorticoids suppress bone formation by attenuating osteoblast differentiation via the monomeric glucocorticoid receptor

Development of osteoporosis severely complicates long-term glucocorticoid (GC) therapy. Using a Cre-transgenic mouse line, we now demonstrate that GCs are unable to repress bone formation in the absence of glucocorticoid receptor (GR) expression in osteoblasts as they become refractory to hormone-induced apoptosis, inhibition of proliferation, and differentiation. In contrast, GC treatment still reduces bone formation in mice carrying a mutation that only disrupts GR dimerization, resulting in bone loss in vivo, enhanced apoptosis, and suppressed differentiation in vitro. The inhibitory GC effects on osteoblasts can be explained by a mechanism involving suppression of cytokines, such as interleukin 11, via interaction of the monomeric GR with AP-1, but not NF-kappaB. Thus, GCs inhibit cytokines independent of GR dimerization and thereby attenuate osteoblast differentiation, which accounts, in part, for bone loss during GC therapy.

Effect of chitosan particles and dexamethasone on human bone marrow stromal cell osteogenesis and angiogenic factor secretion

Chitosan is a polysaccharide scaffold used to enhance cartilage repair during treatments involving bone marrow stimulation, and it is reported to increase angiogenesis and osteogenesis in vivo. Here, we tested the hypotheses that addition of chitosan particles to the media of human bone marrow stromal cell (BMSC) cultures stimulates osteogenesis by promoting osteoblastic differentiation and by favoring the release of angiogenic factors in vitro. Confluent BMSCs were cultured for 3 weeks with 16% fetal bovine serum, ascorbate-2-phosphate and disodium beta-glycerol phosphate, in the absence or presence of dexamethasone, an anti-inflammatory glucocorticoid commonly used as an inducer of BMSC osteoblast differentiation in vitro. As expected, dexamethasone slowed cell division, stimulated alkaline phosphatase activity and enhanced matrix mineralization. Added chitosan particles accumulated intra- and extracellularly and, while not affecting most osteogenic features, they inhibited osteocalcin release to the media at day 14 and interfered with mineralized matrix deposition. Interestingly, dexamethasone promoted cell attachment and suppressed the release and activation of matrix metalloprotease-2 (MMP-2). While chitosan particles had no effect on the release of angiogenic factors, dexamethasone significantly inhibited (p<0.05 to p<0.0001) the release of vascular endothelial growth factor (VEGF), granulocyte-macrophage colony stimulating factor (GM-CSF), tumor necrosis factor-alpha (TNF-alpha), interleukins 1beta, 4, 6, and 10 (IL-1beta, IL-4, IL-6, IL-10), and a host of other inflammatory factors that were constitutively secreted by BMSCs. These results demonstrate that chitosan particles alone are not sufficient to promote osteoblast differentiation of BMSCs in vitro, and suggest that chitosan promotes osteogenesis in vivo through indirect mechanisms. Our data further show that continuous addition of dexamethasone promotes osteoblastic differentiation in vitro partly by inhibiting gelatinase activity and by suppressing inflammatory cytokines which result in increased cell attachment and cell cycle exit.

Toxicity induced enhanced extracellular matrix production in osteoblastic cells cultured on single-walled carbon nanotube networks

A central effort in biomedical research concerns the development of materials for sustaining and controlling cell growth. Carbon nanotube based substrates have been shown to support the growth of different kinds of cells (Hu et al 2004 Nano Lett. 4 507-11; Kalbacova et al 2006 Phys. Status Solidi b 13 243; Zanello et al 2006 Nano Lett. 6 562-7); however the underlying molecular mechanisms remain poorly defined. To address the fundamental question of mechanisms by which nanotubes promote bone mitosis and histogenesis, primary calvariae osteoblastic cells were grown on single-walled carbon nanotube thin film (SWNT) substrates. Using a combination of biochemical and optical techniques we demonstrate here that SWNT networks promote cell development through two distinct steps. Initially, SWNTs are absorbed in a process that resembles endocytosis, inducing acute toxicity. Nanotube-mediated cell destruction, however, induces a release of endogenous factors that act to boost the activity of the surviving cells by stimulating the synthesis of extracellular matrix.

Bone morphogenetic protein-4 induced rat hepatic progenitor cell (WB-F344 cell) differentiation toward hepatocyte lineage

Hepatic progenitor cells are local stem cells in the liver and they can be differentiated into either hepatocytes or cholangiocytes depending on different stimulations. These stimulations include extracellular growth factors and intracellular transcription factors. Bone morphogenetic protein 4 (BMP4) is a member of transforming growth factor beta (TGF-beta) superfamily and was first identified as growth factor to induce ectopic bone formation from skeletal muscle. Role of BMP4 in the liver is still unclear especially its role in hepatic progenitor cells (HPCs) differentiation. BMP4 was used to stimulate rat HPCs (WB-F344 cells) and differentiation of WB-F344 cells was investigated by reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot analysis. Both adenovirus delivered BMP4 and recombinant BMP4 were able to induce expression of hepatocyte markers such as albumin, TAT-1, and G6Pase but not cholangiocyte markers such as beta4-integrin and CK19. BMP4 induced differentiation of WB-F344 cells toward hepatocytes was mediated by increase in phosphorylation of Smad1 and ERK1/2. Moreover, BMP4 also stimulated expression of transcription factor--C/EBP-alpha, which involved in differentiation of WB-F344 cells toward hepatocytes. BMP4 is able to stimulate WB-F344 cells differentiation toward hepatocyte lineage.

Glucocorticoid-dependent Wnt signaling by mature osteoblasts is a key regulator of cranial skeletal development in mice

Glucocorticoids are important regulators of bone cell differentiation and mesenchymal lineage commitment. Using a cell-specific approach of osteoblast-targeted transgenic disruption of intracellular glucocorticoid signaling, we discovered a novel molecular pathway by which glucocorticoids, mainly through the mature osteoblast, regulate the cellular mechanisms that govern cranial skeleton development. Embryonic and neonatal transgenic mice revealed a distinct phenotype characterized by hypoplasia and osteopenia of the cranial skeleton; disorganized frontal, parietal and interparietal bones; increased suture patency; ectopic differentiation of cartilage in the sagittal suture; and disturbed postnatal removal of parietal cartilage. Concurrently, expression of Mmp14, an enzyme essential for calvarial cartilage removal, was markedly reduced in parietal bone and cartilage of transgenic animals. Expression of Wnt9a and Wnt10b was significantly reduced in osteoblasts with disrupted glucocorticoid signaling, and accumulation of beta-catenin, the upstream regulator of Mmp14 expression, was decreased in osteoblasts, chondrocytes and mesenchymal progenitors of transgenic mice. Supracalvarial injection of Wnt3a protein rescued the transgenic cranial phenotype. These results define novel roles for glucocorticoids in skeletal development and delineate how osteoblasts--under steroid hormone control--orchestrate the intricate process of intramembranous bone formation by directing mesenchymal cell commitment towards osteoblastic differentiation while simultaneously initiating and controlling cartilage dissolution in the postnatal mouse.

Mitogen activated protein kinase-dependent inhibition of osteocalcin gene expression by transforming growth factor-beta1

TGF-beta (transforming growth factor-beta) plays a key role in osteoblast differentiation and bone development. While the ability of TGF-beta to inhibit the expression of osteoblast differentiation genes has been well documented, the mechanism of this inhibition is not yet completely characterized. Runx2, a transcription factor necessary for expression of osteoblast differentiation genes is a central target of inhibition by TGF-beta. In this study, we found that TGF-beta1 inhibits expression of osteoblast differentiation genes without altering expression of Runx2. Transient transfection experiments determined that TGF-beta1 inhibited osteocalcin promoter activity and this effect is mediated through Runx2. We further identified that there was no change in protein expression, cellular localization, or DNA binding affinity of Runx2 after TGF-beta1-treatment of osteoblasts, suggesting that Runx2 undergoes post-translational modifications following TGF-beta1 treatment. Co-immunoprecipitation experiments identified increased phosphorylation of Runx2 when differentiating osteoblasts were treated with TGF-beta1. Mitogen activated protein kinase (MAPK) inhibitors relieved the TGF-beta1-inhibitory effect of Runx2-mediated osteocalcin expression. Thus, our results suggest that TGF-beta1-inhibition of osteoblast differentiation is dependent on the MAPK pathway and this effect is most likely mediated by post-translational modification of Runx2 such as phosphorylation rather than other regulatory mechanisms.

Dexamethasone inhibition of confluence-induced apoptosis in human mesenchymal stem cells

Human mesenchymal stem cells (MSCs) have been extensively characterized with respect to their in vitro expansion and differentiation potential, especially with respect to osteogenesis. Dexamethasone (Dex) is a well-known inducer of osteogenic differentiation of MSCs, but little is known about the effect of Dex treatment on apoptosis in MSCs. In this study, apoptosis in MSCs was examined with respect to cell density and Dex supplementation, using DAPI staining and DNA fragmentation ELISA Assay. In MSC cultures initiated at 1.0, 3.0, and 9.0 x 10(3) cells per cm(2), it was found that higher MSC density correlated with increased apoptosis and that this apoptotic effect was diminished in cultures containing 100 nM Dex. MSCs and fibroblasts were co-cultured, along with empty insert controls, and assayed for apoptosis by ELISA and DAPI counts to determine if soluble factors accounted for the cell density-related apoptosis. No difference was seen between MSCs cultured with inserts containing either MSCs, fibroblasts, or empty control. To determine cell contact effects, BrdU-labeled MSCs were cultured alone or with unlabeled chondrocytes at 2x and 8x the number of MSCs, with and without Dex, and apoptosis levels quantified. The results showed increased apoptosis at greater cell densities, and that the amount of apoptosis was greatly diminished in cultures containing Dex. These results show that apoptosis in MSCs is cell density-related, requires direct cell contact, and that Dex treatment reduces or eliminates this density-related apoptosis. These results may impact how MSCs should be cultured for clinical applications.