Mechanisms of glucocorticoid action in bone

Glucocorticoid-induced osteoporosis in growing mice is not prevented by simultaneous intermittent PTH treatment

Glucocorticoids (GCs) are widely used in medicine for treatment of chronic diseases. Especially in children, prolonged treatment causes growth retardation and early onset of osteoporosis. Human parathyroid hormone (PTH) has an anabolic effect on bone when administrated intermittently. The aim of the present study was to examine whether a combined therapy of dexamethasone (DEX) and PTH could prevent the detrimental effects of GC on cortical and trabecular bone in the femur and vertebrae of growing mice. Three-week-old female FVB mice were treated with control, DEX, PTH, or a combination of DEX and PTH by daily subcutaneous injections. After 4 weeks, animals were killed and the femur and L5 vertebra were isolated. Cortical and trabecular bone parameters and relative calcium density were measured by high-resolution X-ray micro-computed tomography (micro-CT). In the femur, PTH can reverse the effects of DEX on bone volume to control. However, it cannot reverse the undermineralization, which most likely is a strong contributor to bone fragility. In the vertebra, PTH improves bone volume to some extent but does not restore the values to normal. It augments the negative effect of DEX on mineralization. We conclude that the detrimental effects of DEX in the growing skeleton cannot be prevented by simultaneous PTH treatment.

Bone disease after renal transplantation

In light of greatly improved long-term patient and graft survival after renal transplantation, improving other clinical outcomes such as risk of fracture and cardiovascular disease is of paramount importance. After renal transplantation, a large percentage of patients lose bone. This loss of bone results from a combination of factors that include pre-existing renal osteodystrophy, immunosuppressive therapy, and the effects of chronically reduced renal function after transplantation. In addition to low bone volume, histological abnormalities include decreased bone turnover and defective mineralization. Low bone volume and low bone turnover were recently shown to be associated with cardiovascular calcifications, highlighting specific challenges for medical therapy and the need to prevent low bone turnover in the pretransplant patient. This Review discusses changes in bone histology and mineral metabolism that are associated with renal transplantation and the effects of these changes on clinical outcomes such as fractures and cardiovascular calcifications. Therapeutic modalities are evaluated based on our understanding of bone histology.

In vitro response of primary human bone marrow stromal cells to recombinant human bone morphogenic protein-2 in the early and late stages of osteoblast differentiation

A number of factors must be added to human bone marrow stromal cells (hBMSCs) in vitro to induce osteogenesis, including ascorbic acid (AA), beta-glycerophosphate (GP), and dexamethasone (Dex). Bone morphogenic protein (BMP)-2 is an osteoinductive factor that can commit stromal cells to differentiate into osteoblasts. However, it is still not clear whether the addition of BMP-2 alone in vitro can induce hBMSCs to complete osteoblast differentiation, resulting in matrix mineralization. This study compares the effects of BMP-2 and Dex, alone and combined, on the early and late stages of hBMSC differentiation. We found that BMP-2 causes a significant induction of alkaline phosphatase (ALP) activity in hBMSCs, with a transcriptional upregulation of known BMP-2-responsive genes, and the stable expression of cbfa1 in the nucleus and the regions surrounding the nucleus in the early phase of osteoblast differentiation. However, continuous treatment with BMP-2 alone at doses ranging from 100 to 300 ng/mL results in a less efficient enhancement of in vitro matrix mineralization, despite a significant induction of ALP activity at a concentration of 100 ng/mL. Our results reflect how the effects of BMP-2 on hBMSCs can vary depending on the stage of osteoblast differentiation, and highlight the need to understand the role of BMP-2 in primary hBMSCs derived from diverse sources in order to increase the efficiency of using BMP-2 in osteoinductive therapies.

Dexamethasone downregulates the expression of parathyroid hormone-related protein (PTHrP) in mesenchymal stem cells

Parathyroid hormone-related protein (PTHrP) has been shown to have anabolic effects in women with postmenopausal osteoporosis. PTHrP promotes the recruitment of osteogenic cells and prevents apoptotic death of osteoblasts and osteocytes. The receptor responsible for the effects of PTHrP is the common PTH/PTHrP receptor (PTH1R). Glucocorticoids (GC) are commonly used as drugs to treat inflammatory diseases. Long-term GC treatments are often associated with bone loss which can lead to GC-induced osteoporosis. The aim of this work was to study the effects of the glucocorticoid dexamethasone (Dex) on the expression of PTHrP and PTH1R in adult human mesenchymal stem cells, the progenitor cells of osteoblasts. Adult human mesenchymal stem cells (hMSC) were cultured and differentiated by standard methods. The expression of PTHrP and PTH1R mRNA was assayed by real-time qPCR. The PTHrP release into the culture media was measured by an immunoradiometric assay. Treatment with Dex (10 nM) resulted in an 80% drop in the PTHrP release within 6 h. A 24 h Dex treatment also reduced the expression of PTHrP mRNA by up to 90%. The expression of PTH1R receptor mRNA was simultaneously increased up to 20-fold by 10 nM Dex. The effects of Dex on PTHrP and PTH1R were dose-dependent and experiments with the GC-receptor antagonist mifepristone showed an involvement of GC-receptors in these effects. In addition to the Dex-induced effects on PTHrP and PTH1R, Dex also increased mineralization and the expression of the osteoblast markers Runx2 and alkaline phosphatase. In our studies, we show that dexamethasone decreases the expression of PTHrP and increases the expression of the PTH1R receptor. This could have an impact on PTHrP-mediated anabolic actions on bone and could also affect the responsiveness of circulating PTH. The results indicate that glucocorticoids affect the signalling pathway of PTHrP by regulating both PTHrP and PTH1R expression and these mechanisms could be involved in glucocorticoid-induced osteoporosis.

Transient dynamic actin cytoskeletal change stimulates the osteoblastic differentiation

Dynamic cytoskeletal changes appear to be one of intracellular signals that control cell differentiation. To test this hypothesis, we examined the effects of short-term actin cytoskeletal changes on osteoblastic differentiation. We found an actin polymerization interfering reagent, cytochalasin D, promoted osteoblastic differentiation in mouse preosteoblastic MC3T3-E1 cells. We also found that these effects were mediated by the protein kinase D (PKD) pathway. Short-term cytochalasin D treatment increased alkaline phosphatase (ALP) activity, osteocalcin (OCN) secretion, and mineralization of the extracellular matrix in MC3T3-E1 cells, with temporary changes in actin cytoskeleton. Furthermore, the disruption of actin cytoskeleton induced phosphorylation of 744/748 serine within the activation loop of PKD in a dose-dependent manner. The protein kinase C (PKC)/PKD inhibitor Go6976 suppressed cytochalasin D-induced acceleration of osteoblastic differentiation, whereas Go6983, a specific inhibitor of conventional PKCs, did not. Involvement of PKD signaling was confirmed by using small interfering RNA to knock down PKD. In addition, another actin polymerization interfering reagent, latrunculin B, also stimulated ALP activity and OCN secretion with PKD activation. On the other hand, the present data suggested that transient dynamic actin cytoskeletal reorganization could be a novel cellular signal that directly stimulated osteoblastic differentiation.

Validation of the sheep as a large animal model for the study of vertebral osteoporosis

Rats have long been the animal of choice for research in the field of osteoporosis. In the search for a complementary large animal model the sheep appears useful but hitherto the extent of bone loss from the spine has failed to reach a level that is generally accepted as osteoporotic in humans. Osteoporosis was induced in ten sheep using ovariectomy, low calcium diet and steroid injection for 6 months. Bone samples of iliac crest (IC), lumbar spine (LS), and proximal femur (PF) from the osteoporotic sheep were compared with those from four normal sheep using densitometry, histomorphometry, biochemistry and basic mechanical testing. The differences were examined using an analysis of variance with Tukey-Kramer test. Overall, the bone mineral density at LS and PF decreased more than 25% after treatment. Trabecular bone volume decreased by 29.2, 33.4 and 42.6% in IC, LS and PF, respectively. The failure load of the LS in axial compression was reduced to 2,003 from 6,140 N. The extent of bone loss was sufficient to categorise these sheep as osteoporotic although the pattern of bone loss varied between sites. Reduced mechanical competence in LS confirmed the suitability of this model for evaluation of potential treatments for osteoporosis.

Isolation of osteoprogenitors from murine bone marrow by selection of CD11b negative cells

Selection of cells having the most osteogenic potential is a strategy used in bone tissue engineering. Preclinical studies using murine bone marrow cells must consider the large amount of hematopoietic cells in the adherent fraction. The aim of this study was to enrich a murine bone marrow cell population with osteoprogenitors by using a simple and reliable method. Bone marrow from C57Bl/6 mice was extracted and cells which adhered onto plastic were expanded in primary culture for 14 days. Immunolabeling of the CD11b surface antigen was performed and the CD11b(-) cell fraction was isolated by FACS. Sorted and unsorted populations were analyzed for gene expression of osteoblast differentiation, alkaline phosphatase (AlkP) activity and matrix mineralization capacities. Selection of CD11b(-) cells increased the number of AlkP(+) cells from the plastic adherent fraction from 6.3% +/- 0.8 to 56% +/- 3.3 with a sevenfold increase in AlkP activity. mRNA analysis revealed a significant increase in the CD11b(-) fraction for Osterix (41-fold), RANKL (17-fold), M-CSF (8-fold) and Runx-2 (8-fold). An osteogenic population was obtained with improved capacities to produce a mineralized extracellular matrix in vitro, independently of the presence of glucocorticoids in the culture medium.

The biological roles of extracellular and intracytoplasmic glucocorticoids in skeletal cells

Osteoporosis is the most common metabolic disease characterized by loss of the normal density of bone, resulting in fragile bone and a higher risk of fractures. Patients under glucocorticoids treatment are susceptible to glucocorticoid-induced osteoporosis (GIO). The normal bone turnover depends on a balance between osteoblasts and osteoclasts. The skeletal cells including osteoblasts, osteoclasts, osteocytes and their precursors demonstrate altered features while they are cocultured with different extracellular glucocorticoids, or their intracytoplasmic glucocorticoids modified by genetic manipulation of 11beta-HSD isozyme. However, recent studies have also demonstrated different or even contradictive outcomes on whether the glucocorticoids inhibit or increase biological activity of these skeletal cells. Focusing on the roles of extracellular glucorticoids, intracytoplasmic glucocorticoids and the mechanism of transmembrane passage of the glucocorticoids, this review reveals that glucocorticoids may exert either inhibitive or enhancing influence on these skeletal cells, but relying on the difference in cell origins, methodology, and types of glucocorticoids. In addition, the effects of glucocorticoids may be dose- and time-dependent.

Retroviral-mediated gene therapy for the differentiation of primary cells into a mineralizing osteoblastic phenotype

Bone tissue engineering has emerged as a promising strategy for the repair of critical-sized skeletal fractures. However, the clinical application of this approach has been limited by the availability of a robust mineralizing cell source. Non-osteogenic cells, such as skin fibroblasts, are an attractive cell-source alternative because they are easy to harvest from autologous donor skin biopsies and display a high capacity for in vitro expansion. We have recently demonstrated that retroviral gene delivery of the osteoblastic transcription factor Runx2/Cbfa1 promotes osteogenic differentiation in primary dermal fibroblasts cultured in monolayer. Notably, sustained expression of Runx2 was not sufficient to promote functional osteogenesis in these cells, and co-treatment with the steroid hormone dexamethasone was required to induce deposition of biologically-equivalent matrix mineralization. On the basis of these results, we then investigated the osteogenic capacity of these genetically engineered fibroblasts when seeded on polymeric scaffolds in vitro and in vivo. These experiments demonstrated that Runx2-expressing fibroblasts seeded on collagen scaffolds produce significant levels of matrix mineralization after 28 days in vivo implantation in a subcutaneous, heterotopic site. Overall, these results offer evidence that transcription factor-based gene therapy may be a powerful strategy for the conversion of a non-osteogenic cellular phenotype into a mineralizing cell source for bone repair applications. This concept may also be applied to control functional differentiation in a broad range of cell types and tissue engineering applications. The chapter below outlines detailed methods for the isolation and ex vivo genetic modification of primary dermal fibroblasts using retroviral-mediated delivery of the Runx2 transgene in both monolayer culture and three-dimensional scaffolds.

BMP-2 vs. BMP-4 expression and activity in glucocorticoid-arrested MC3T3-E1 osteoblasts: Smad signaling, not alkaline phosphatase activity, predicts rescue of mineralization

Pharmacological glucocorticoids (GCs) inhibit bone formation, leading to osteoporosis. GCs inhibit bone morphogenetic protein-2 (Bmp2) expression, and rhBMP-2 restores mineralization in GC-arrested osteoblast cultures. To better understand how GCs regulate BMPs, we investigated Bmp transcription, as well as rhBMP-induced Smad and alkaline phosphatase (ALP) activity. Bmp2 cis-regulatory regions were analyzed by reporter plasmids and LacZ-containing bacterial artificial chromosomes. We found that GCs inhibited Bmp2 via a domain > 50 kb downstream of the coding sequence. Bmp expression was evaluated by RT-PCR; whereas GCs strongly inhibited Bmp2, Bmp4 was abundantly expressed and resistant to GCs. Both rhBMP-2 and rhBMP-4 restored mineralization in GC-arrested cultures; rhBMP-2 was 5-fold more effective when dosing was based on ALP activation, however, the rhBMPs were equipotent when dosing was based on Smad transactivation. In conclusion, GCs regulate Bmp2 via a far-downstream domain, and activation of Smad, not ALP, best predicts the pro-mineralization potential of rhBMPs.

Skeletal involvement in adult patients with endogenous hypercortisolism

Overt endogenous glucocorticoid excess is a well-recognized cause of bone loss and osteoporotic fractures. Cortisol excess inhibits bone formation, increases bone resorption, impairs calcium absorption from the gut, and affects the secretion of several hormones (in particular gonadotropins and GH), cytokines, and growth factors, influencing bone metabolism. The glucocorticoid excess mainly affects trabecular bone, leading to vertebral fractures in up to 70% of patients. Osteoporotic fractures may be the presenting symptom of an otherwise silent glucocorticoid excess and can precede the diagnosis of hypercortisolism by up to 2 yr. The removal of glucocorticoid excess leads to a recovery of bone mass which is, however, often incomplete and delayed, although it reduces the risk of osteoporotic fractures. Bisphosphonate therapy has been suggested to be useful in maintaining bone mass in these patients. Subclinical hypercortisolism, a condition of impaired hypothalamic- adrenal-axis homeostasis without the classical signs and symptoms of glucocorticoid excess, is a recently defined entity, which has been shown to be associated to increased bone resorption, bone loss, and high prevalence of vertebral fractures regardless of gonadal status. However, data about the effect of this subtle glucocorticoid excess on bone are still scarce and conflicting. Moreover, it is not yet known whether removing the cause of subclinical hypercortisolism leads to a recovery of bone mass and reduces the risk of osteoporotic fractures. Finally, recent data suggest that subclinical hypercortisolism is a common and underrated finding in patients with established osteoporosis. In summary, it is crucial to evaluate the risk of osteoporosis and fractures in patients with glucocorticoid excess; on the other hand, it also seems advisable to screen for glucocorticoid excess patients with osteoporotic fractures without known secondary causes of osteoporosis.

Glucocorticoids alter craniofacial development and increase expression and activity of matrix metalloproteinases in developing zebrafish (Danio rerio)

Teratogenic effects are observed following long-term administration of glucocorticoids, although short-term glucocorticoid therapy is still utilized to reduce fetal mortality, respiratory distress syndrome, and intraventricular hemorrhage in preterm infants. However, the mechanism of glucocorticoid-induced teratogenicity is unknown. We hypothesize that glucocorticoid-induced teratogenesis is mediated through the glucocorticoid receptor (GR) and results from altering the expression and activity of the matrix metalloproteinases (MMPs). During embryogenesis, degradation of the extracellular matrix to allow for proper cellular migration and tissue organization is a tightly regulated process requiring appropriate temporal and spatial expression and activity of the MMPs. Studies have demonstrated that MMP gene expression can be either inhibited or induced by glucocorticoids in a variety of model systems. Using the zebrafish (Danio rerio) as a model of development, the data presented here demonstrate that embryonic exposure to the glucocorticoids dexamethasone or hydrocortisone increased expression of two gelatinases, MMP-2 ( approximately 1.5-fold) and MMP-9 (7.6- to 9.0-fold), at 72 h postfertilization (hpf). Further, gelatinase activity was increased approximately threefold at 72 hpf following glucocorticoid treatment, and changes in craniofacial morphogenesis were also observed. Cotreatment of zebrafish embryos with each glucocorticoid and the GR antagonist RU486 resulted in attenuation of glucocorticoid-induced increases in MMP expression (52-84% decrease) and activity (41-94% decrease). Furthermore, the abnormal craniofacial phenotype observed following glucocorticoid exposure was less severe following RU486 cotreatment. These studies demonstrate that in the embryonic zebrafish, dexamethasone, and hydrocortisone alter expression and activity of MMP-2 and -9, and suggest that these increases may be mediated through the GR.

Sex-specific association of the glucocorticoid receptor gene with extreme BMD

To study the role of the GR gene on BMD regulation in the Chinese, a sex-specific association study was performed. The results indicated that GR variation contributed to the extreme BMD variation in the Chinese.

INTRODUCTION

The glucocorticoid (GC) receptor (GR) gene is an important candidate gene for BMD regulation in GC-induced osteoporosis (GIO). However, no study has explored the genetic effects of the GR gene on BMD variation in the Chinese population.

MATERIALS AND METHODS

Our sample consisted of 800 unrelated subjects (400 women and 400 men) with extreme age-adjusted hip BMD Z-scores selected from a population composed of 1988 normal adult Chinese Han. Four single nucleotide polymorphisms (SNPs) in the GR gene were genotyped. Both single SNP and haplotype association analyses were conducted.

RESULTS

SNP rs1866388 (p(c) = 0.028) was found to be significantly associated with extreme BMD only in men. In both sexes, haplotypes involving rs1866388 and rs2918419 were found to have different frequency distributions in extremely low and high BMD groups (p(p) = 0.024, 0.001, and 0.002 in women and 0.002, 0.003, and 0.003 in men for window sizes of two, three, and four SNPs, respectively). Most shared haplotypes showed opposite effects between women and men.

CONCLUSIONS

For the first time, our study suggested the possible role of the GR gene on BMD regulation and sex specificity in the association of GR with extreme BMD in the Chinese.

Recombinant human platelet-derived growth factor: biology and clinical applications

The abilities of bone to remodel, fractures to repair, and bone grafts to incorporate are all fundamental reflections of the bone remodeling cycle. This process is characterized by the recruitment and differentiation of osteoblastic and osteoclastic cell populations, whose cellular activities are coordinated and regulated by an elaborate system of growth factors and cytokines. One of the crucial biological factors responsible for reparative osseous activity is platelet-derived growth factor (PDGF). The potent stimulatory effects of PDGF as a chemoattractant and mitogen for mesenchymal cells (including osteogenic cells), along with its ability to promote angiogenesis, have been demonstrated in a variety of preclinical models predicting maxillofacial, spine and appendicular skeletal, and soft-tissue applications. The biological profile of PDGF, including its ability to recruit osteoprogenitor cells, makes it particularly suited to address the skeletal defects that are seen with comorbid conditions such as osteoporosis, diabetes, and the effects of smoking. The clinical success and safety that have been demonstrated with use of recombinant human PDGF (rhPDGF) in the repair of periodontal defects have led to U.S. Food and Drug Administration (FDA) approval of rhPDGF for this indication. Ongoing pilot and pivotal trials in the United States and internationally will continue to clarify the promising role of PDGF in the treatment of challenging skeletal disorders.

Serum fasting cortisol in relation to bone, and the role of genetic variations in the glucocorticoid receptor

OBJECTIVE

To examine the relationship between endogenous cortisol and bone, and the role of genetic variations in the glucocorticoid receptor (GR).

DESIGN AND PATIENTS

The Longitudinal Ageing Study Amsterdam (LASA), a population-based cohort study in older men and women.

MEASUREMENTS

Serum fasting cortisol was assessed by competitive immunoassay (n = 1214); bone mineral density (BMD) by dual X-ray absorptiometry (DXA) (n = 502); broadband ultrasound attenuation (BUA) by ultrasound (n = 1209); fractures by self-report (n = 1211); and GR gene polymorphisms (ER22/23EK, N363S, 9beta, BclI) were genotyped by Taqman (n = 858).

RESULTS

Higher serum fasting cortisol was significantly associated with lower BMD at all sites and BUA at the heel in women, although most relationships were attenuated by age and body mass index (BMI). The effect on femoral neck BMD remained statistically significant in the fully adjusted model (r = -0.135, P = 0.04). No significant associations in men were found. Female 9beta G-allele carriers had 50.2 nmol/l lower cortisol and 1.2 lower free cortisol levels than AA homozygotes [P = 0.01 for (free) cortisol]. Furthermore, female BclI GG homozygotes had 54.8 nmol/l higher cortisol levels than C-carriers (P = 0.03). In the total population, BclI GG homozygotes had 0.05 g/cm(2) lower trochanteric region BMD (P = 0.03). For the other GR gene polymorphisms, no significant associations were found.

CONCLUSIONS

Higher cortisol levels are associated with lower femoral neck BMD in elderly women. The G allele of the 9beta polymorphism was associated with lower serum cortisol levels in women. Female BclI GG homozygotes had higher serum cortisol levels, and BclI GG homozygotes had lower trochanteric region BMD in the total population.

Vanadate prevents glucocorticoid-induced apoptosis of osteoblasts in vitro and osteocytes in vivo

Skeletal mass is maintained by a balance between formation and resorption, cell proliferation and apoptosis. In vitro, glucocorticoids (GCs) decrease extracellular signal-regulated kinases (ERK) activation by mitogens, thus inhibiting osteoblast proliferation. Both ERK activity and proliferation are restored by co-treatment with the protein tyrosine phosphatase inhibitor, vanadate. Since ERK signalling may also be anti-apoptotic, we explored the effects of vanadate on GC-induced apoptosis in vitro and in vivo. Apoptosis in MBA-15.4 pre-osteoblasts increased from 6 h and remained up to eightfold higher through 6 days of 10(- 6) M dexamethasone (Dex) treatment. Co-incubation with 10(- 7) M vanadate markedly reduced apoptosis at all time points. Vanadate also prevented GC-induced poly-ADP-ribose polymerase cleavage. We assessed the transcriptional profiles of seven anti-apoptotic proteins (Bcl-2, Bcl-X(L), inhibitors of apoptosis protein-1 (IAP-1), IAP-2, X-linked IAP (XIAP), Fas-associated death-domain-like IL-1beta-converting enzyme-inhibitory protein (FLIP(Long)) and FLIP(Short)) in osteoblasts subjected to various stimuli using real-time quantitative PCR. Although these anti-apoptotic genes responded to different mitogenic conditions, Dex failed to repress their expression, and in fact significantly up-regulated Bcl-X(L), IAP-2 and XIAP. Dex may therefore induce apoptosis by up-regulating pro-apoptotic gene expression. We have previously demonstrated that rats treated with GC develop low formation osteoporosis (bone histomorphometry and DEXA) and skeletal fragility (breaking strength) that were largely prevented by co-treatment with vanadate. We report here that vertebrae from rats treated with 3.5 mg/kg per day methylprednisolone for 9 weeks showed increased incidence of terminal deoxynucleotidyl transferase-mediated biotin-dUTP nick end-labelling-positive apoptotic osteocytes, which was reduced by vanadate co-treatment. We conclude that vanadate prevents GC-induced apoptosis of pre-osteoblasts in vitro and osteocytes in vivo, and this may contribute to its bone-sparing effects in vivo.

l-Carnitine protects against apoptosis of murine MC3T3-E1 osteoblastic cells

L-carnitine (LC), an amino acid with a major role in cellular energy metabolism, has positive effects on bone metabolism. However, the effect of LC on apoptosis of osteoblast in vitro has not been reported. The aim of this study was to investigate the action of LC on apoptosis of mouse osteoblastic cell line MC3T3-E1. Cell apoptosis was measured by sandwich-enzyme-immunoassay. Release of cytochrome c from mitochondria into cytosol and Bcl-2, Bax protein levels were determined by Western blot analysis. The enzyme substrate was used to assess the activation of caspase-3 and caspase-9. LC inhibited MC3T3-E1 cell apoptosis induced by serum deprivation. Our study also shows that LC decreased cytochrome c release and caspase-3 and caspase-9 activation in serum-deprived MC3T3-E1 cells. Furthermore, LC protected against MC3T3-E1 cell apoptosis induced by the glucocorticoid (GC) dexamethasone (Dex).

Risedronate prevents the loss of microarchitecture in glucocorticoid-induced osteoporosis in rats

Osteoporosis is a severe complication of glucocorticoid treatment. Bisphosphonates are a powerful therapeutic option to prevent osteoporotic fractures. The aims of this study were: a) to determine bone alterations induced by therapy with glucocorticoids (GC); b) to establish the efficacy of risedronate (Ris) in the prevention of these effects. We studied 40 female Sprague-Dawley rats randomly divided into 4 groups of treatment, administered 3 times a week sc: 1.

CONTROL

vehicle of methylprednisolone (GC) + vehicle of Ris; 2. Ris: Ris 5 mug/kg body weight vehicle of GC; 3. GC: GC 7 mg/kg + vehicle of Ris; 4. GC+Ris: GC 7 mg/kg, Ris 5 microg/kg. Animals were treated for 30 days and then were sacrificed. Densitometry was performed at baseline and at the end of the treatment. Right tibiae were removed for histomorphometric analyses. The GC group showed a 7% decrease in bone density vs controls (p<0.05), while the GC+Ris group was associated with a 3.5% increase in bone density vs controls (p<0.05). In the GC group, histomorphometric evaluations showed reduced bone volume (BV/TV) and thinning of trabeculae (Tb.Th) vs controls (BV/TV: 31+/-1 vs 35+/-1%, p<0.05; Tb.Th: 43+/-2 vs 50+/-3 microm, p<0.01; Ac.f: 1.8+/-0.2 vs 1.6+/-0.3 N/yr). The GC+Ris group had increased BV/TV and Tb.Th, and reduced Ac.f vs the GC group. Ris also maintained trabecular microarchitecture. At the histological level, glucocorticoid-induced osteoporosis was characterized by decreased bone volume, reduced osteoblastic activity, and deterioration of microarchitecture. Ris counteracted these effects both by prolonging osteoblast activity, and by maintaining bone microarchitecture.

Quantifying osteoblast and osteocyte apoptosis: challenges and rewards

Since the initial demonstration of the phenomenon in murine and human bone sections approximately 10 yr ago, appreciation of the biologic significance of osteoblast apoptosis has contributed greatly not only to understanding the regulation of osteoblast number during physiologic bone remodeling, but also the pathogenesis of metabolic bone diseases and the pharmacology of some of the drugs used for their treatment. It is now appreciated that all major regulators of bone metabolism including bone morphogenetic proteins (BMPs), Wnts, other growth factors and cytokines, integrins, estrogens, androgens, glucocorticoids, PTH and PTH-related protein (PTHrP), immobilization, and the oxidative stress associated with aging contribute to the regulation of osteoblast and osteocyte life span by modulating apoptosis. Moreover, osteocyte apoptosis has emerged as an important regulator of remodeling on the bone surface and a critical determinant of bone strength, independently of bone mass. The detection of apoptotic osteoblasts in bone sections remains challenging because apoptosis represents only a tiny fraction of the life span of osteoblasts, not unlike a 6-mo-long terminal illness in the life of a 75-yr-old human. Importantly, the phenomenon is 50 times less common in human bone biopsies because human osteoblasts live longer and are fewer in number. Be that as it may, well-controlled assays of apoptosis can yield accurate and reproducible estimates of the prevalence of the event, particularly in rodents where there is an abundance of osteoblasts for inspection. In this perspective, we focus on the biological significance of the phenomenon for understanding basic bone biology and the pathogenesis and treatment of metabolic bone diseases and discuss limitations of existing techniques for quantifying osteoblast apoptosis in human biopsies and their methodologic pitfalls.

Glucocorticoid receptor action in beneficial and side effects of steroid therapy: lessons from conditional knockout mice

Glucocorticoids (GCs) are potent immune suppressive drugs with unfortunately severe side effects. Different molecular modes of actions of the GC receptor (GR) have been identified. Transcriptional transactivation by binding of a dimerized GR protein complex to the promoter of GC regulated genes or interference with activity of pro-inflammatory transcription factors by GR monomers are considered as the two major mechanisms. It has been hypothesized that selective GR agonists (SEGRAs) addressing dimer-independent function would reveal potent steroid therapeutic activity with reduced side effects. Recent studies of a mouse knock-in strain with a dimerization-deficient GR demonstrate that some inflammatory processes can be suppressed by GCs, while others cannot. Also side effects of GCs occur in these mice. Thus, depending on the process that is treated, SEGRA could be therapeutically more or less effective and not all side effects of steroid therapy may be reduced.

Matrix metalloproteinase-13 is required for zebra fish (Danio rerio) development and is a target for glucocorticoids

Matrix metalloproteinases (MMPs) are endopeptidases that degrade the proteins of the extracellular matrix (ECM). Expression and activity of the MMPs are essential for embryogenesis, where MMPs participate in the normal ECM remodeling that occurs during tissue morphogenesis and development. Studies have demonstrated that MMP gene expression is inhibited by glucocorticoids in mammalian cell culture systems and that exposure to glucocorticoids causes developmental abnormalities in several species. Therefore, we proposed that glucocorticoids impede normal development through alteration of MMP expression. Zebra fish (Danio rerio) were used as a model to study MMP-13 expression both during normal embryogenesis and following acute exposure to two glucocorticoids, dexamethasone, and hydrocortisone. MMP-13 is one of three collagenases identified in vertebrates that catalyzes the degradation of type I collagens at neutral pH. MMP-13 expression varied during zebra fish development, with peak expression at 48 h post-fertilization (hpf). Morpholino knockdown studies showed that MMP-13 expression is necessary for normal zebra fish embryogenesis. Acute exposure to dexamethasone and hydrocortisone resulted in abnormal zebra fish development including craniofacial abnormalities, altered somitogenesis, blood pooling and pericardial and yolk sac edema as well as increased MMP-13 mRNA and activity at 72 hpf. In situ hybridization experiments were used to confirm the increase in MMP-13 expression following glucocorticoid treatment and showed elevated MMP-13 expression in the rostral trunk, brain, eye, heart, and anterior kidney of treated embryos. These data demonstrate that normal zebra fish embryogenesis requires MMP-13 and that dexamethasone and hydrocortisone modulate the expression of this gene, leading to increased activity and potentially contributing to subsequent dysmorphogenesis.

Inductive effects of dexamethasone on the mineralization and the osteoblastic gene expressions in mature osteoblast-like ROS17/2.8 cells

We examined the effects of dexamethasone (Dex), a synthetic glucocorticoid, on the formation of mineralized bone nodules and the gene expressions of the late osteoblastic markers, bone sialoprotein (BSP), osteocalcin (OC), and osteopontin (OPN) in mature osteoblast ROS17/2.8 cells. Treatment of ROS17/2.8 cells with Dex resulted in the induction of mineralization accompanied with increasing BSP and OC expressions. Previous reports have demonstrated that BSP and OC expressions are regulated by Runx2. Then, we hypothesized that Dex might promote osteoblastic differentiation and mineralization on ROS17/2.8 by Runx2. In this study, no effect was observed in mRNA and protein expression of Runx2. However, the transcriptional activity of Runx2 was enhanced by Dex treatment. Furthermore, the Dex-induced BSP and OC expressions decreased after the transfection of Runx2 small-interfering RNAs (siRNAs). These results suggested that the enhancement of Runx2 transcriptional activity by Dex treatment may be followed by the activation of osteoblast marker genes, such as BSP and OC to thereby produce a bone-specific matrix that subsequently becomes mineralized.

A juvenile case of Cushing's disease incidentally discovered with multiple bone fractures

A 19-year-old woman who had decreased eight centimeters in stature was diagnosed as Cushing's disease with multiple spine compression fractures. At the age of 18, the patient had a complex fracture and gradually presented the features of Cushing's syndrome. Her plasma ACTH and cortisol levels were extremely high. Radiological findings and chemical markers for bone metabolism showed severe osteoporosis. Magnetic resonance imaging showed the presence of a pituitary microadenoma. After transsphenoidal surgery was performed, subsequently all endocrine data improved. This case indicates that Cushing's syndrome should be considered for severe osteoporotic juvenile patients.

The effects of dexamethasone and dehydroepiandrosterone (DHEA) on cytokines and receptor expression in a human osteoblastic cell line: potential steroid-sparing role for DHEA

Osteoporosis and associated fractures are the most common and debilitating complication of glucocorticoid use. The use of alternative anti-inflammatory agents without the deleterious skeletal effects of glucocorticoids is needed. Dehydroepiandrosterone (DHEA) may have immunomodulatory as well as positive effects on bone. For our further understanding of the mechanisms of action of DHEA, as a steroid-sparing agent, we investigated and compared the effects of dexamethasone (DEX) and DHEA on the regulation of the downstream effector pathway of osteoclastogenesis; RANKL/OPG and a range of inflammatory/pro-resorbing cytokines and receptors using a human clonal osteoblastic cell line. The cells were treated with DEX, DHEA, and androstenedione (ANDI). The mRNA expression of RANKL and OPG was determined by real-time PCR after overnight incubation. The regulation of a broad spectrum of cytokines by DEX and DHEA was also investigated using a human cytokine/growth factor and receptor gene array consisting of 268 cytokine-related cDNAs. To confirm some of the gene expression changes, protein production was measured by ELISA. RANKL expression and RANKL/OPG ratio were increased by DEX. This effect was reversed by co-treatment with both DHEA or ANDI. Several pro-inflammatory/resorptive cytokines including IL-6, IL-4, IFN-gamma, macrophage inhibitory factor (MIF) were down-regulated not only by DEX but also by DHEA. In contrast to DEX, DHEA did not lead to suppression of growth factors including vascular endothelial growth factor (VEGF), fibroblast growth factor-5 (FGF-5), insulin-like growth factor-binding protein3 (IGF-BP3). Several new target genes previously documented to influence bone formation were up-regulated by DHEA such as Notch 2, insulin receptor, thrombin receptor (PAR1). The data suggest that DHEA has immunomodulatory properties without the catabolic effects on bone remodeling, observed with glucocorticoid use. DHEA may thus prove useful as a steroid-sparing agent in the management of inflammatory disorders such as SLE or rheumatoid arthritis. Further in vivo studies are indicated.

High dose glucocorticoid hampers bone formation and resorption after bone marrow ablation in rat

We analyzed the effect of glucocorticoid on bone regeneration after bone marrow ablation in tibiae of 8-week-old rats. Methylprednisolone sodium succinate (MPSS) was injected intramuscularly at a dose of 100 mg/kg/day for 3 days. Tibiae on days 1, 3, 5, 7, 10, 12, and 14 after ablation were subjected to tartrate-resistant acid phosphatase staining, immunohistochemistry, in situ hybridization, and transmission electron microscopy (TEM), and measurement of the volume of newly-formed bone and the osteoclast number. MPSS significantly decreased the newly-formed bone volume on day 7, and immature bone still remained on day 10 in the MPSS-treated group. The volume of this bone was significantly higher than that in the control group. However, there were no differences between the groups in the osteoclast number, the expression of mRNAs for osteoblast differentiation markers, and alkaline phosphatase and cathepsin K judged by immunohistochemistry. TEM findings showed no difference in the form of osteoblasts, whereas osteoclasts in the MPSS-treated group had less developed ruffled borders, compared to those in the control group. These results suggest that MPSS treatment affects neither the differentiation nor the shape of osteoblasts, and does not change the osteoclast number or the cathepsin K level. However, high dose MPSS inhibits both bone formation and resorption during bone regeneration after rat tibial bone marrow ablation, and inhibits ruffled border formation in osteoclasts. These data will be useful to develop bone regenerative therapies for bone diseases due to high dose steroid administration.

Alendronate or alfacalcidol in glucocorticoid-induced osteoporosis

BACKGROUND

Treatment with glucocorticoids is associated with bone loss starting soon after therapy is initiated and an increased risk of fracture.

METHODS

We performed a randomized, double-placebo, double-blind clinical trial of 18 months' duration among patients with a rheumatic disease who were starting glucocorticoids at a daily dose that was equivalent to at least 7.5 mg of prednisone. A total of 201 patients were assigned to receive either alendronate (10 mg) and a placebo capsule of alfacalcidol daily or alfacalcidol (1 microg) and a placebo tablet of alendronate daily. The primary outcome was the change in bone mineral density of the lumbar spine in 18 months; the secondary outcome was the incidence of morphometric vertebral deformities.

RESULTS

A total of 100 patients received alendronate, and 101 received alfacalcidol; 163 patients completed the study. The bone mineral density of the lumbar spine increased by 2.1 percent in the alendronate group (95 percent confidence interval, 1.1 to 3.1 percent) and decreased by 1.9 percent in the alfacalcidol group (95 percent confidence interval, -3.1 to -0.7 percent). At 18 months, the mean difference of change in bone mineral density between the two groups was 4.0 percent (95 percent confidence interval, 2.4 to 5.5 percent). Three patients in the alendronate group had a new vertebral deformity, as compared with eight patients in the alfacalcidol group (of whom three had symptomatic vertebral fractures) (hazard ratio, 0.4; 95 percent confidence interval, 0.1 to 1.4).

CONCLUSIONS

During this 18-month trial in patients with rheumatic diseases, alendronate was more effective in the prevention of glucocorticoid-induced bone loss than was alfacalcidol. (ClinicalTrials.gov number, NCT00138983 [ClinicalTrials.gov].).

Impaired cortical bone acquisition and osteoblast differentiation in mice with osteoblast-targeted disruption of glucocorticoid signaling

To determine the role of endogenous glucocorticoids in bone, we previously developed transgenic mice in which a 2.3 kb fragment of the Col1a1 promoter drives 11beta-hydroxysteroid dehydrogenase 2 expression in mature osteoblasts. This transgene should inactivate glucocorticoids upstream of all receptor signaling pathways. In the present study, we show that femoral cortical bone area and thickness were approximately 10-15% lower in transgenic mice than in wild-type littermates. Femur length was unchanged, indicating that bone elongation was not affected in this model. Expression of osteocalcin mRNA, pOBCol2.3-GFP (a green fluorescent protein marker of mature osteoblasts), and the formation of mineralized nodules were impaired in ex vivo transgenic primary calvarial cultures. The extent of crystal violet staining in bone marrow cultures, indicative of the number of adherent stromal cells, was also decreased. These data suggest that endogenous glucocorticoids are required for cortical bone acquisition and full osteoblast differentiation. It appears that blocking glucocorticoid signaling in vivo leads to a decrease in the commitment and/or expansion of progenitors entering the osteoblast lineage.

Post-renal transplantation hypophosphatemia: a review and novel insights

PURPOSE OF REVIEW

This review intends to elucidate the pathophysiologic mechanism of renal phosphorus loss in the post-renal transplantation population. This review will provide new insight in to the pathophysiologic mechanism(s) responsible for the development of this phenomenon and will also explore the pathogenetic role of persistent phosphorus wasting in the development of post-renal transplantation osteodystrophy.

RECENT FINDINGS

Recently, the phosphaturic hormone, fibroblast growth factor-23, has been ascertain to be increased in the sera of patients with chronic kidney and end-stage renal disease. There is new evidence that a non-PTH humoral factor is persistently present in post-renal transplantation patients that is likely responsible for the observed persistent renal phosphorus loss. We offer that fibroblast growth factor-23 (and/or other phosphatonins) is the culprit factor responsible for the phenomenon of persistent hypophosphatemia in post-renal transplantation patients. Moreover, we believe that the phenomenon of persistent renal phosphorus wasting is an important but overlooked cause of osteodystrophy and increased fracture risk in this patient population.

SUMMARY

The pathophysiology of post-renal transplantation phosphorus wasting is complex and to date is still not fully recognized. Further studies of the regulatory mechanism of fibroblast growth factor-23 and its metabolism may offer additional insights into phosphorus homeostasis and its clinical application in the post-renal transplantation population.

Antigenic Profile of Osteoblasts Present in Human Bone Tissue Sections

The antigenic profile of human osteoblasts was previously analyzed by our group using primary cultures as study samples. These studies suggested a novel functional approach to this cell population. Osteoblasts have a characteristic antigenic profile and share antigens in common with other cell populations that also originate in the bone marrow. Some of the detected antigens are constitutively expressed, while others are modulated by different factors and/or cytokines. The aim of the present study was to analyze the antigens present in osteoblasts in vivo, since the presence of certain biomolecules in fetal bovine serum may modulate the antigenic expression, compromising the results. For this purpose, human bone tissue sections were analyzed with a wide panel of mAbs and using the immunoperoxidase technique. CD10, CD44 and alkaline phosphatase antigens and IL-12, IL-18 and IFNγ cytokines were detected in osteoblasts in the bone tissue. However, CD80 and HLA-DR antigens were not found in all samples and when present their expression was weak. The expression of CD54 antigen was moderate or weak. These results allow data obtained by the primary culture of osteoblast-like cells to be endorsed.

Glucocorticoid-induced osteogenesis is negatively regulated by Runx2/Cbfa1 serine phosphorylation

Glucocorticoid hormones have complex stimulatory and inhibitory effects on skeletal metabolism. Endogenous glucocorticoid signaling is required for normal bone formation in vivo, and synthetic glucocorticoids, such as dexamethasone, promote osteoblastic differentiation in several in vitro model systems. The mechanism by which these hormones induce osteogenesis remains poorly understood. We demonstrate here that the coordinate action of dexamethasone and the osteogenic transcription factor Runx2/Cbfa1 synergistically induces osteocalcin and bone sialoprotein gene expression, alkaline phosphatase activity, and biological mineral deposition in primary dermal fibroblasts. Dexamethasone decreased Runx2 phosphoserine levels, particularly on Ser125, in parallel with the upregulation of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) through a glucocorticoid-receptor-mediated mechanism. Inhibition of MKP-1 abrogated the dexamethasone-induced decrease in Runx2 serine phosphorylation, suggesting that glucocorticoids modulate Runx2 phosphorylation via MKP-1. Mutation of Ser125 to glutamic acid, mimicking constitutive phosphorylation, inhibited Runx2-mediated osteoblastic differentiation, which was not rescued by dexamethasone treatment. Conversely, mutation of Ser125 to glycine, mimicking constitutive dephosphorylation, markedly increased osteoblastic differentiation, which was enhanced by, but did not require, additional dexamethasone supplementation. Collectively, these results demonstrate that dexamethasone induces osteogenesis, at least in part, by modulating the phosphorylation state of a negative-regulatory serine residue (Ser125) on Runx2. This work identifies a novel mechanism for glucocorticoid-induced osteogenic differentiation and provides insights into the role of Runx2 phosphorylation during skeletal development.

Regulation of bone mass by Wnt signaling

Wnt proteins are a family of secreted proteins that regulate many aspects of cell growth, differentiation, function, and death. Considerable progress has been made in our understanding of the molecular links between Wnt signaling and bone development and remodeling since initial reports that mutations in the Wnt coreceptor low-density lipoprotein receptor-related protein 5 (LRP5) are causally linked to alterations in human bone mass. Of the pathways activated by Wnts, it is signaling through the canonical (i.e., Wnt/beta-catenin) pathway that increases bone mass through a number of mechanisms including renewal of stem cells, stimulation of preosteoblast replication, induction of osteoblastogenesis, and inhibition of osteoblast and osteocyte apoptosis. This pathway is an enticing target for developing drugs to battle skeletal diseases as Wnt/beta-catenin signaling is composed of a series of molecular interactions that offer potential places for pharmacological intervention. In considering opportunities for anabolic drug discovery in this area, one must consider multiple factors, including (a) the roles of Wnt signaling for development, remodeling, and pathology of bone; (b) how pharmacological interventions that target this pathway may specifically treat osteoporosis and other aspects of skeletal health; and (c) whether the targets within this pathway are amenable to drug intervention. In this Review we discuss the current understanding of this pathway in terms of bone biology and assess whether targeting this pathway might yield novel therapeutics to treat typical bone disorders.

Comparison of Calcitonin versus Calcitonin + Resistance Exercise as Prophylaxis for Osteoporosis in Heart Transplant Recipients

BACKGROUND

Rapid bone loss occurs early after heart transplantation. There is no standard therapeutic intervention to prevent osteoporosis in heart transplant recipients (HTR). The purpose of this study was to determine the effectiveness of a regimen combining the antiresorptive properties of nasal calcitonin with the osteogenic stimulus of resistance exercise.

METHODS

Eighteen candidates for heart transplantation were randomly assigned either to a group that received calcitonin and participated in 6 months of resistance exercise (n=10) or to a group that received only calcitonin (n=8). Calcitonin therapy (200 IU daily for 8 months) was initiated 48 hr after transplantation. Resistance exercise was initiated 2 months after transplantation. Bone mineral density (BMD) of the total body, femur neck, and lumbar vertebra (L2-3) were assessed before, and at 2 and 8 months after transplantation.

RESULTS

Total body and femur neck BMD did not decrease (P>or=0.05) below pretransplantation values at 2 months after transplantation in either group. BMD of the lumbar spine was significantly (P<or=0.05) and comparably decreased at 2 months after transplantation in the calcitonin (-10.1+/-1.8%) and calcitonin + training groups (-12.9+/-2.7%). At 8 months after transplantation lumbar BMD was -16.9% below pretransplant values in the calcitonin group. In contrast, lumbar BMD was restored to within 5% of pretransplant levels in the calcitonin + training group.

CONCLUSIONS

Calcitonin attenuates BMD loss in the total body and femur neck but not in trabecular bone of the lumbar vertebra. Mechanical loading, through progressive resistance exercise, is an osteogenic stimulus in HTR.

The essential role of glucocorticoids for proper human osteoblast differentiation and matrix mineralization

Glucocorticoids (GCs) exert profound effects on bone and are essential for human osteoblast differentiation. However, GCs are still interpreted as negative regulators of bone formation, mainly caused by the detrimental effects on bone after clinical use of GCs. In this paper we emphasize the importance of GCs for proper human osteoblast differentiation and matrix mineralization. We show that human osteoblast differentiation needs to be triggered by GCs in a specific time-window during the early stages of development. Exposure to GCs in the beginning of osteoblast development induces a dose dependent increase in alkaline phosphatase activity and matrix mineralization. GC-induced differentiation stimulated expression of genes involved in bone formation and suppressed genes that negatively regulate bone formation and mineralization. Furthermore we highlight the importance of local cortisol activation in osteoblasts by expression of 11beta-hydroxysteroid dehydrogenase 1 (11beta-HSD1).

Expression of bone morphogenetic proteins in normal human intramembranous and endochondral bones

Bone morphogenetic proteins (BMPs) are growth and differentiation factors that have been purified and widely accepted to be the most important regulators in the processes of bone formation. The aim of this study was to identify the BMPs that are expressed in normal human bone, and to investigate the specific pattern of BMP2-BMP9 expression in normal human intramembranous and endochondral bone to maintain homeostasis, as well as in ex vivo primary cell culture of human osteoblasts from intramembranous and endochondral bone. Semi-quantitative RT-PCR indicated that 2 types of bone of different embryological origin have distinct patterns of BMP expression. BMP3, 4, 7 and 8 were strongly expressed in normal intramembranous bone compared to endochondral bone, whereas BMP2 and 5 were highly expressed in endochondral bone. The expression of BMP9 and BMP15 in human bone was identified for the first time. From the very similar expression patterns of BMPs in fresh normal bone and ex vivo osteoblastic cell culture, it can be proposed that the different proportions of BMPs in normal human intramembranous and endochondral bone needed to maintain normal homeostasis.

Factors other than glucocorticoids are involved in the osteoblast activity decrease caused by tissue injury

OBJECTIVE

Serum osteocalcin is a marker of bone formation. The concentration of osteocalcin is decreased with tissue injury. As glucocorticoids are known both to be increased in this situation and to diminish serum osteocalcin, we have hypothesized that they could be involved in this decrease.

DESIGN AND PATIENTS

We compared osteocalcin levels in two groups of patients undergoing abdominal surgery, one receiving thiopental, and the other etomidate, a glucocorticoid synthesis blocker. For comparative reasons, another protein decreased by glucocorticoids (osteoprotegerin) was measured in patients anaesthetized with thiopental.

MEASUREMENTS

Serum osteocalcin, cortisol and albumin were determined before and over the 24 h following surgery. Serum concentration of osteoprotegerin (OPG) and receptor activator of the nuclear factor kappaB ligand (RANKL) were also determined before and 24 h after surgery in a third group of nine patients who received thiopental for anaesthetic induction.

RESULTS

Cortisol levels were increased in the thiopental group, whereas, as expected, were decreased in etomidate patients. However, serum osteocalcin concentration decreased in a similar way in both groups. Serum OPG and RANKL levels were within the normal range at baseline and did not significantly change after surgery.

CONCLUSIONS

The decrease in serum osteocalcin induced by tissue injury is independent of the increase in cortisol secretion triggered by the latter. In addition, another pharmacologically proven effect of cortisol on bone metabolism, OPG inhibition, could not be demonstrated in the first 24 h following surgery, in spite of the physiological increase in endogenous cortisol secretion taking place in this period.

High-dose glucocorticoid treatment induces rapid loss of trabecular bone mineral density and lean body mass

A recent large-scale study revealed that glucocorticoid treatment increased fracture risk, which occurred at a far smaller dose and by a shorter duration than previously thought. To study the underlying mechanism for the increased risk of fracture, we studied the early changes in bone mineral density (BMD) and body composition by dual energy X-ray absorptiometry (DXA) after initiating high-dose glucocorticoid treatment. High-dose glucocorticoid treatment was arbitrarily defined as daily doses of >or=40 mg of a predonisolone equivalent. The 33 patients enrolled in this study had not received glucocorticoid treatment before. Only 2 months of treatment resulted in substantial BMD loss, most markedly in the lumbar spine, followed by the femoral neck and total body, which suggests the preferential trabecular bone loss. Body composition was also greatly affected. Thus, 2-month treatment with glucocorticoid significantly reduced bone mineral content (BMC), lean body mass (LBM) and increased fat mass (FAT). Our results are likely to have some clinical relevance. First, BMD loss occurs quite rapidly after starting glucocorticoid treatment, and patients receiving glucocorticoid treatment should be more carefully monitored for their BMD. Second, LBM, which mainly represents muscle volume, decreases rapidly after initiating glucocorticoid treatment. Decreased LBM might be also responsible for the increased risk of fracture, since falling is a well-known risk factor for fracture, and patients receiving glucocorticoid treatment should also be evaluated for their body composition.

Inhibition of cortical and cancellous bone formation in glucocorticoid-treated OVX sheep

INTRODUCTION

Little is known about the effects of steroids on cortical bone. The purpose of this study was to explore the effects of glucocorticoid treatment on cortical bone density, strength, and formation of new bone in ovariectomized sheep and to compare it to cancellous bone.

MATERIALS AND METHODS

Sixteen ovariectomized merino sheep either received a 6-month glucocorticoid treatment (GLU; 0.45 mg/kg/day Methylprednisolone s.c.) or were left untreated (control). After 2 and 4 months, newly formed bone was labeled by the administration of calceingreen and Tetracycline-hydrochloride. After 12 months, the animals were sacrificed and cortical specimens were obtained from the tibial diaphysis. Cancellous bone specimens were harvested from the proximal tibia. All specimens were scanned for apparent BMD by pQCT and tested mechanically. Formation of new bone was analyzed using histological slices of the femoral condyles and cross-sections of the mid-diaphysis of the tibia.

RESULTS

The intracortical formation of new bone in glucocorticoid-treated sheep was 70% lower after 2 months and 80% lower after 4 months. Six months after the termination of the steroid administration, the active bone area was 20% lower than in the control group. Cortical width and cortical bone area were reduced by 7-8% and marrow area increased by 8% in steroid-treated animals compared to control animals. Neither cortical apparent bone mineral density nor biomechanical parameters of cortical bone specimens differed between the groups. Cancellous bone formation in steroid-treated animals declined by 68% after 2 months and by 90% after 4 months. After 1 year, cancellous bone formation was reduced by 38% compared to control. The apparent cancellous bone mineral density and cancellous bone compressive stiffness were reduced by 34% and 55%, respectively.

CONCLUSION

A six-month glucocorticoid treatment of ovariectomized sheep resulted in a substantial reduction of bone formation both in cortical and in cancellous bone and reduced cortical width by increased endosteal resorption. Beyond changes in cancellous bone, impaired cortical bone remodeling may add to the increased fracture risk at the hip or the distal radius in patients treated with steroids.

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.

Estrogen protects primary osteocytes against glucocorticoid-induced apoptosis

Glucocorticoid-induced osteoporosis may be at least in part due to the increased apoptosis of osteocytes. To study the role of osteocyte apoptosis in glucocorticoid-induced osteoporosis, we isolated primary osteocytes from murine calvaria for the analysis of the effects of dexamethasone in in vitro culture. The cells were identified by morphology, cytochemical staining, immunocytochemical staining and mRNA expression of phosphate-regulating gene with homology to endopeptidases on the X chromosome (PHEX) and sclerosteosis/van Buchem disease gene (SOST). We found that dexamethasone induced osteocyte apoptosis in a dose-dependent manner. A glucocorticoid receptor antagonist, mifepristone (RU486), suppressed dexamethasone-induced osteocyte apoptosis, suggesting that it was mediated by glucocorticoid receptor. Immunocytochemical stainings showed that glucocorticoid receptors are present in primary osteocytes, and they were translocated to nuclei after the exposure to dexamethasone. Addition of estrogen prevented glucocorticoid receptor translocation into nuclei. Corresponding antiapoptotic effects in primary osteocytes were also seen after the pretreatment of primary osteocytes with a picomolar concentration of estrogen. The pure antiestrogen ICI 182,780 inhibited estrogen effect on apoptosis induced by dexamethasone. These data suggest that glucocorticoid receptors play an important role in glucocorticoid-induced osteocyte apoptosis. Most importantly, estrogen has a protective effect against osteocyte apoptosis. To conclude, the mechanism of glucocorticoid-induced osteoporosis may be due to the apoptosis of osteocytes, which can be opposed by estrogen.

Histomorphometric analysis of glucocorticoid-induced osteoporosis

Bone histomorphometry or quantitative histology consists of counting or measuring tissue components: cells, extracellular constituents and microarchitecture. Bone histomorphometry is the only method that allows the measurement of mineralization rate and the study of bone formation at three levels: cell, remodeling unit and tissue levels. It is a useful tool to explain the pathogenesis and cellular mechanisms of different metabolic bone diseases such as glucocorticoid-induced osteoporosis (GIO). Glucocorticoids (GC) affect calcium and bone metabolism at every level, but the main effect is the osteoblastic dysfunction. Concerning the bone formation, some histomorphometric studies have shown a depressed osteoblastic activity at a cell, bone remodeling unit, and tissue levels. In addition, there is evidence of a shortening of the period in which the osteoblasts work actively forming the bone matrix. This latter effect seems to occur after high cumulative doses of GC. With regard to the resorption, the results are still debated, but histomorphometric parameters seem to be increased in the majority of studies, at least in the first period of the GC treatment. From a structural point of view, GC seem to induce a thinning of the trabeculae without their perforation, which occurs only after high cumulative doses. Anti-resorptive treatments, such as bisphosphonates, are able to counteract the negative effects of GC on bone. In particular, along with their active working period, they prolong the lifespan of osteoblasts and osteocytes. In addition, the anti-resorptive treatments seem to extend the time for secondary mineralization through a reduction of the Activation Frequency. The latter is an intriguing mechanism of bisphosphonates in GIO that needs further ad hoc investigations.

Osteoporosis associated with excess glucocorticoids

Excess glucocorticoids, whether endogenous or exogenous, can cause osteoporosis and fractures. Even low doses of oral glucocorticoids and mild endogenous hypercortisolism may be associated with bone loss. Patients treated with glucocorticoids, however, often are not evaluated and treated for this problem. Patients on chronic glucocorticoids or initiating these drugs should have their bone density measured and appropriate laboratory studies. They should be treated with adequate calcium and vitamin D, and antiresorptive therapy (particularly bisphosphonates) should be considered.

Pharmacokinetics and pharmacodynamics of systemically administered glucocorticoids

Glucocorticoids have pleiotropic effects that are used to treat diverse diseases such as asthma, rheumatoid arthritis, systemic lupus erythematosus and acute kidney transplant rejection. The most commonly used systemic glucocorticoids are hydrocortisone, prednisolone, methylprednisolone and dexamethasone. These glucocorticoids have good oral bioavailability and are eliminated mainly by hepatic metabolism and renal excretion of the metabolites. Plasma concentrations follow a biexponential pattern. Two-compartment models are used after intravenous administration, but one-compartment models are sufficient after oral administration.The effects of glucocorticoids are mediated by genomic and possibly nongenomic mechanisms. Genomic mechanisms include activation of the cytosolic glucocorticoid receptor that leads to activation or repression of protein synthesis, including cytokines, chemokines, inflammatory enzymes and adhesion molecules. Thus, inflammation and immune response mechanisms may be modified. Nongenomic mechanisms might play an additional role in glucocorticoid pulse therapy. Clinical efficacy depends on glucocorticoid pharmacokinetics and pharmacodynamics. Pharmacokinetic parameters such as the elimination half-life, and pharmacodynamic parameters such as the concentration producing the half-maximal effect, determine the duration and intensity of glucocorticoid effects. The special contribution of either of these can be distinguished with pharmacokinetic/pharmacodynamic analysis. We performed simulations with a pharmacokinetic/pharmacodynamic model using T helper cell counts and endogenous cortisol as biomarkers for the effects of methylprednisolone. These simulations suggest that the clinical efficacy of low-dose glucocorticoid regimens might be increased with twice-daily glucocorticoid administration.