Insulin-like growth factor binding proteins in femoral and vertebral bone marrow stromal cells: expression and regulation by thyroid hormone and dexamethasone

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

Thyroid hormone actions in cartilage and bone

Thyroid hormones exert widespread and complex actions in almost all tissues during development, throughout childhood and in adults. The skeleton is an important T3-target tissue that exemplifies these processes, and yet understanding of the specific cellular and molecular mechanisms of T3 action in bone and cartilage remains incomplete. Here, the skeleton is considered as a T3-target tissue. The actions of thyroid hormones during skeletal development and in chondrocytes and growth plate cartilage during post-natal linear growth are outlined. The physiological importance of these actions are discussed in relation to patients with autosomal dominant mutations in genes encoding the thyroid hormone receptors TRα1 and TRβ, and in mice harbouring deletions or mutations of the orthologous genes. The role of thyroid hormones and the control of T3 action in bone turnover and maintenance are also outlined, and T3 action in bone-forming osteoblasts and bone-resorbing osteoclasts discussed. The physiological and functional consequences of T3 action in bone are considered in relation to mutant mouse models and to effects on bone mineral density and fracture susceptibility in humans. Finally, new studies identifying a putative role for thyroid hormone metabolism in articular cartilage maintenance and the pathogenesis of osteoarthritis are considered. The pharmacological context of these new findings is discussed, emphasising the importance of this emerging field of study in thyroid hormone pathophysiology.

Biological significance of a thyroid hormone-regulated secretome

The thyroid hormone, 3,3,5-triiodo-L-thyronine (T3), modulates several physiological processes, including cellular growth, differentiation, metabolism and proliferation, via interactions with thyroid hormone response elements (TREs) in the regulatory regions of target genes. Several intracellular and extracellular protein candidates are regulated by T3. Moreover, T3-regulated secreted proteins participate in physiological processes or cellular transformation. T3 has been employed as a marker in several disorders, such as cardiovascular disorder in chronic kidney disease, as well as diseases of the liver, immune system, endocrine hormone metabolism and coronary artery. Our group subsequently showed that T3 regulates several tumor-related secretory proteins, leading to cancer progression via alterations in extracellular matrix proteases and tumor-associated signaling pathways in hepatocellular carcinomas. Therefore, elucidation of T3/thyroid hormone receptor-regulated secretory proteins and their underlying mechanisms in cancers should facilitate the identification of novel therapeutic targets. This review provides a detailed summary on the known secretory proteins regulated by T3 and their physiological significance. This article is part of a Special Issue entitled: An Updated Secretome.

Mechanisms of action of thyroid hormones in the skeleton

BACKGROUND

Thyroid hormones regulate skeletal development, acquisition of peak bone mass and adult bone maintenance. Abnormal thyroid status during childhood disrupts bone maturation and linear growth, while in adulthood it results in altered bone remodeling and an increased risk of fracture

SCOPE OF REVIEW

This review considers the cellular effects and molecular mechanisms of thyroid hormone action in the skeleton. Human clinical and population data are discussed in relation to the skeletal phenotypes of a series of genetically modified mouse models of disrupted thyroid hormone signaling.

MAJOR CONCLUSIONS

Euthyroid status is essential for normal bone development and maintenance. Major thyroid hormone actions in skeletal cells are mediated by thyroid hormone receptor α (TRα) and result in anabolic responses during growth and development but catabolic effects in adulthood. These homeostatic responses to thyroid hormone are locally regulated in individual skeletal cell types by the relative activities of the type 2 and 3 iodothyronine deiodinases, which control the supply of the active thyroid hormone 3,5,3'-L-triiodothyronine (T3) to its receptor.

GENERAL SIGNIFICANCE

Population studies indicate that both thyroid hormone deficiency and excess are associated with an increased risk of fracture. Understanding the cellular and molecular basis of T3 action in skeletal cells will lead to the identification of new targets to regulate bone turnover and mineralization in the prevention and treatment of osteoporosis. This article is part of a Special Issue entitled Thyroid hormone signaling.

Culture media for the differentiation of mesenchymal stromal cells

Mesenchymal stromal cells (MSCs) can be isolated from various tissues such as bone marrow aspirates, fat or umbilical cord blood. These cells have the ability to proliferate in vitro and differentiate into a series of mesoderm-type lineages, including osteoblasts, chondrocytes, adipocytes, myocytes and vascular cells. Due to this ability, MSCs provide an appealing source of progenitor cells which may be used in the field of tissue regeneration for both research and clinical purposes. The key factors for successful MSC proliferation and differentiation in vitro are the culture conditions. Hence, we here summarize the culture media and their compositions currently available for the differentiation of MSCs towards osteogenic, chondrogenic, adipogenic, endothelial and vascular smooth muscle phenotypes. However, optimal combination of growth factors, cytokines and serum supplements and their concentration within the media is essential for the in vitro culture and differentiation of MSCs and thereby for their application in advanced tissue engineering.

Transcriptional activation of insulin-like growth factor binding protein 6 by 17beta-estradiol in SaOS-2 cells

Osteoblasts can synthesize the insulin-like growth factors (IGFs) and the IGF-binding proteins (IGFBPs), which may either enhance or attenuate IGF-stimulated bone cell proliferation. Since estrogen induced osteoblastic differentiation and proliferation through an estrogen-responsive gene in target cells, we investigated the effects of estrogen on IGFBP-6 expression in the human osteoblastic-like cell line SaOS-2. Expressions of IGFBP-6 protein and mRNA increased 2.8 and 2-fold, respectively, in the presence of 17-beta-estradiol (E2) (0.01 to 1 micronM) and estrogen receptor (ER) in SaOS-2 cells. On the other hand, E2 induced a 2-fold increase in SaOS-2 cell proliferation. To identify genomic sequences associated with estrogen responsiveness, the 5'-promoter region (-44 to +118) of the IGFBP-6 gene was cloned into a chloramphenicol acetyltransferase (CAT) reporter vector. E2 induced a 3-fold increase in CAT activity in SaOS-2 cells transiently transfected with this construct. Identification of the estrogen-responsive element (ERE) [5-CCTTCA CCTG-3] (-9 to +1) in this IGFBP-6 gene promoter region was confirmed using electromobility shift assays and deletion analysis. This functional ERE was important for E2-induced trans-activation of the IGFBP-6 gene. These results demonstrate that E2 exhibits a positive effect on IGFBP-6 gene transcription through estrogen-liganded ER binding to the functional ERE in the IGFBP-6 gene promoter in SaOS-2 cells.

Synovium-derived stem cell-based chondrogenesis

Synovium is considered a candidate source of cells for cartilage tissue engineering. Compared with mesenchymal stem cells (MSCs) from other sources, synovium-derived stem cells (SDSCs) have a higher capacity for chondrogenic differentiation. Our objective was to define cocktails of growth factors that support the growth and chondrogenic differentiation of SDSCs in chemically defined medium. We established a fast and highly selective technique of negative isolation of SDSC populations. The individual and combined effects of three growth factors-transforming growth factor-beta1 (TGF-beta1), insulin-like growth factor I (IGF-I), and basic fibroblast growth factor (FGF-2)-were evaluated in serum-free pellet cultures of SDSCs for the chondrogenesis of SDSCs using histology, biochemical analysis, and real-time RT-PCR. In vitro studies identified TGF-beta1 as the key factor for both the growth and chondrogenesis of SDSCs. The highest rates of SDSC growth were observed with the synergistic interaction of all three factors. With respect to chondrogenic differentiation of SDSCs, the interaction of TGF-beta1 and IGF-I applied simultaneously was superior to the sequential application of these two factors or any other combination of growth factors studied. Based on these findings, we propose a two-step protocol for the derivation of chondrogenic SDSCs: a cocktail of TGF-beta1, IGF-I, and FGF-2 is applied first to induce cell growth followed by a cocktail of TGF-beta1 and IGF-I applied to induce chondrogenesis.

Insulin-like growth factor I releasing silk fibroin scaffolds induce chondrogenic differentiation of human mesenchymal stem cells

Growth factor releasing scaffolds are an emerging alternative to autologous or allogenous implants, providing a biologically active template for tissue (re)-generation. The goal of this study is to evaluate the feasibility of controlled insulin-like growth factor I (IGF-I) releasing silk fibroin (SF) scaffolds in the context of cartilage repair. The impact of manufacturing parameters (pH, methanol treatment and drug load) was correlated with IGF-I release kinetics using ELISA and potency tests. Methanol treatment induced water insolubility of SF scaffolds, allowed the control of bioactive IGF-I delivery and did not affect IGF-I potency. The cumulative drug release correlated linearly with the IGF-I load. To evaluate the chondrogenic potential of the scaffolds, hMSC were seeded on unloaded and IGF-I loaded scaffolds in TGF-beta supplemented medium. Chondrogenic differentiation of hMSC was observed on IGF-I loaded scaffolds, starting after 2 weeks and more strongly after 3 weeks, whereas no chondrogenic responses were observed on unloaded control scaffolds. IGF-I loaded porous SF scaffolds have the potential to provide chondrogenic stimuli to hMSC. Evidence for in vivo cartilage (re)generation must be demonstrated by future, pre-clinical proof of concept studies.

Thyroid and bone

This article provides a summary of the numerous interactions between the thyroid gland and the skeleton, in the normal state, in disorders of thyroid function and as a result of thyroid malignancy. It recaps the current understanding of bone growth and development in the endochondral growth plate and the normal mechanisms of mature bone remodeling. The actions of thyroid hormones on these processes are described, and the clinical impact of thyroid disorders and their treatments on the bone are summarized. Finally, our current understanding of the physiology of bone metastases from thyroid cancer is covered.

Effects of insulin-like growth factor binding proteins in bone -- a matter of cell and site

The actions of the insulin-like growth factor (IGF)-system are controlled by six IGF-binding proteins (IGFBPs). The IGFBPs are thought to affect local effects of IGF-I and IGF-II due to higher affinity if compared to IGF-I receptors and due to cell-type specific IGFBP expression patterns. It was found in IGFBP knockout models that the IGFBP family is functionally redundant. Thus, functional analysis of potential effects of IGFBPs is dependent on descriptive studies and models of IGFBP overexposure in vitro and in vivo. In the literature, the role of the IGFBPs for bone growth is highly controversial and, to date, no systematic look has been taken at IGFBPs resolving functional aspects of IGFBPs at levels of cell types and specific locations within bones. Since IGFBPs are thought to represent local modulators of the IGF actions and also exert IGF-independent effects, this approach is particularly reasonable on a physiological level. By sorting the huge number of in part controversial results on IGFBP effects in bone present in the literature for distinct cell types and bone sites it is possible to generate a focused, more specific and a less controversial picture of IGFBP functions in bone.

Fine mapping dissects pleiotropic growth quantitative trait locus into linked loci

A recurring issue in studies of quantitative trait loci (QTLs) is whether QTLs that appear to have pleiotropic effects are indeed caused by pleiotropy at single loci or by linked QTLs. Previous work identified a QTL that affected tail length in mice and the lengths of various bones, including the humerus, ulna, femur, tibia, and mandible. The effect of this QTL on tail length has since been found to be due to multiple linked QTLs and so its apparently pleiotropic effects may have been due to linked QTLs with distinct effects. In the present study we examined a line of mice segregating only for a 0.94-Mb chromosomal region known to contain a subset of the QTLs influencing tail length. We measured a number of skeletal dimensions, including the lengths of the skull, mandible, humerus, ulna, femur, tibia, calcaneus, metatarsus, and a tail bone. The QTL region was found to have effects on the size of the mandible and length of the tail bone, with little or no effect on the other traits. Using a randomization approach, we rejected the null hypothesis that the QTL affected all traits equally, thereby demonstrating that the pleiotropic effects reported earlier were due to linked loci with distinct effects. This result underlines the possibility that seemingly pleiotropic effects of QTLs may frequently be due to linked loci and that high-resolution mapping will often be required to distinguish between pleiotropy and linkage.

Low cortical bone density measured by computed tomography in children and adolescents with untreated hyperthyroidism

OBJECTIVE

To determine whether increased thyroid hormones levels have an effect on various bone components (cortical vs cancellous bone).

STUDY DESIGN

The anthropometric and 3-dimensional quantitative computed tomography (CT) bone measurements, including bone density (BD), cross-sectional area (CSA) of the lumbar spine and femur, and cortical bone area (CBA) of the femur, of 18 children and adolescents with untreated hyperthyroidism were reviewed and compared with those of age-, sex-, and ethnicity-matched historical controls.

RESULTS

No significant differences in height, weight, body mass index (BMI), or pubertal staging between patients and controls were found. Cortical BD was significantly lower (P < .001) in children and adolescents with hyperthyroidism compared with historical controls. After adjusting for weight and height, no difference in femur CSA between hyperthyroid children and historical controls was evident. No significant correlations among thyroid hormone levels, antithyroid antibody levels, and cortical BD values were found.

CONCLUSIONS

As determined by CT, cortical bone is the preferential site of bone loss in children and adolescents with untreated hyperthyroidism.

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.

Effect of IGF-I in the chondrogenesis of bone marrow mesenchymal stem cells in the presence or absence of TGF-beta signaling

A novel role for IGF-I in MSC chondrogenesis was determined. IGF-I effects were evaluated in the presence or absence of TGF-beta signaling by conditionally inactivating the TGF-beta type II receptor. We found that IGF-I had potent chondroinductive actions on MSCs. IGF-I effects were independent from and additive to TGF-beta.

INTRODUCTION

Mesenchymal stem cells (MSCs) can be isolated from adult bone marrow (BM), expanded, and differentiated into several cell types, including chondrocytes. The role of IGF-I in the chondrogenic potential of MSCs is poorly understood. TGF-beta induces MSC chondrogenic differentiation, although its actions are not well defined. The aim of our study was to define the biological role of IGF-I on proliferation, chondrogenic condensation, apoptosis, and differentiation of MSCs into chondrocytes, alone or in combination with TGF-beta and in the presence or absence of TGF-beta signaling.

MATERIALS AND METHODS

Mononuclear adherent stem cells were isolated from mouse BM. Chondrogenic differentiation was induced by culturing high-density MSC pellets in serum- and insulin-free defined medium up to 7 days, with or without IGF-I and/or TGF-beta. We measured thymidine incorporation and stained 2-day-old pellets with TUNEL, cleaved caspase-3, peanut-agglutinin, and N-cadherin. Seven-day-old pellets were measured in size, stained for proteoglycan synthesis, and analyzed for the expression of collagen II and Sox-9 by quantitative real time PCR. We obtained MSCs from mice in which green fluorescent protein (GFP) was under the Collagen2 promoter and determined GFP expression by confocal microscopy. We conditionally inactivated the TGF-beta type II receptor (TbetaRII) in MSCs using a cre-lox system, generating TbetaRII knockout MSCs (RIIKO-MSCs).

RESULTS AND CONCLUSIONS

IGF-I modulated MSC chondrogenesis by stimulating proliferation, regulating cell apoptosis, and inducing expression of chondrocyte markers. IGF-I chondroinductive actions were equally potent to TGF-beta1, and the two growth factors had additive effects. Using RIIKO-MSCs, we showed that IGF-I chondrogenic actions are independent from the TGF-beta signaling. We found that the extracellular signal-related kinase 1/2 mitogen-activated protein kinase (Erk1/2 MAPK) pathway mediated the TGF-beta1 mitogenic response and in part the IGF-I proliferative action. Our data, by showing the role of IGF-I and TGF-beta1 in the critical steps of MSC chondrogenesis, provide critical information to optimize the therapeutic use of MSCs in cartilage disorders.

Intracellular pH as a determinant of vascular smooth muscle function

Intracellular pH (pHi) is a physiological parameter that is intimately linked to contractility, growth and proliferation of vascular smooth muscle (VSM). Regarding contractility, no general unifying concept of pHi regulation but a rather complex relation between pHi signals and vascular tone has been revealed so far. The modulation of vasotone by pHi depends on the type of blood vessel as well as on the pattern of regulatory input signals. In addition, changes in pHi have been recognized as an important cellular signal to determine the fate of cells in terms of proliferation or apoptosis. Cellular sensors for pHi include a variety of ion transport systems which control intracellular Ca2+ gradients and are likely to serve as a link between pHi and cell functions. Here we provide an overview on the potential targets and mechanisms that transduce pHi signals in VSM. The role of pHi-sensing signaling complexes and localized pHi signaling as the basis of diversity of pHi regulation of VSM function is discussed.

Insulin-like Growth Factor-I Induces Early Osteoblast Gene Expression in Human Mesenchymal Stem Cells

Human adult mesenchymal stem cells (hMSCs) differentiate into an osteogenic lineage if the appropriate differentiative cues, such as dexamethasone or bone morphogenetic protein 2 (BMP-2), are present. This study was undertaken to determine the role of insulin-like growth factor I (IGFI) in the regulation of early osteoblast differentiation in hMSC. Previous studies have shown that IGF-I, regulates bone formation and remodeling by participating in the differentiation of mature cells of osteoblast lineage. We hypothesized that IGF-I exerted its effects early, but the effects were too subtle to be detected. Therefore, engineered hMSCs to produce IGF-I via adenoviral transfection and used quantitative real-time PCR (qPCR) to assess marker gene expression. Here we show that IGF-I up-regulates Type I collagen, Runx2, and alkaline phosphatase (Alp) gene expression in hMSCs, genes indicative of early osteogenic differentiation. We also observed mineral deposition in the absence of dexamethasone (Dex) in hMSC cultures treated with recombinant human BMP-2 after transduction with Ad-IGF-I. In conclusion Igf-I transduction up-regulated markers of osteoblastic differentiation and in conjunction with recombinant BMP-2-induced matrix mineralization independently of Dex (see Salasznyk et al., Stem Cells Dev 14(6):608-620, 2005, this issue).

A frequent regulatory variant of the estrogen-related receptor alpha gene associated with BMD in French-Canadian premenopausal women

Genes are important BMD determinants. We studied the association of an ESRRA gene functional variant with BMD in 1335 premenopausal women. The ESRRA genotype was an independent predictor of L2-L4 BMD, with an effect similar to smoking and equivalent to a 10-kg difference in weight.

INTRODUCTION

Several genetic polymorphisms have been associated with osteoporosis or osteoporosis fractures, but no functional effect has been shown for most of these gene variants. Because functional studies have implicated estrogen-related receptor alpha (ESRRA) in bone metabolism, we evaluated whether a recently described regulatory variant of the ESRRA gene is associated with lumbar and hip BMD as measured by DXA and with heel bone parameters as measured by quantitative ultrasound (QUS).

MATERIALS AND METHODS

Heel bone parameters were measured by right calcaneal QUS in 1335 healthy French-Canadian premenopausal women, and one-half of these women also had their BMD evaluated at two sites: femoral neck and lumbar spine (L2-L4) by DXA. All bone measures were tested separately for association with the ESRRA genotype by analysis of covariance. The significance of the ESRRA contribution to the model was also assessed by two different permutation tests.

RESULTS

A statistically significant association between ESRRA genotype and lumbar spine BMD was observed: women carrying the long ESRRA genotype had a 3.9% (0.045 g/cm2) higher lumbar spine BMD than those carrying the short ESRRA genotype (p = 0.004), independently of other risk factors measured. This effect of ESRRA genotype is similar to the effect of smoking and equivalent to a 10-kg difference in weight. This association was confirmed by permutation tests (p = 0.004). The same trend was observed for femoral neck BMD (2.6%, p = 0.07). However, no association was observed between ESRRA and QUS heel bone measures.

CONCLUSION

These results support the genetic influence of this ESRRA regulatory variant on BMD.

Pregnancy-associated plasma protein-A proteolytic activity in rat vertebral cell cultures: Stimulation by dexamethasone-a potential mechanism for glucocorticoid regulation of osteoprogenitor proliferation and differentiation

Glucocorticoids (GCs) at physiological concentrations stimulate osteoprogenitor proliferation and differentiation in rat bone cell populations, and this is mediated in part by an increased response to insulin-like growth factors (IGFs). Since IGF binding proteins (IGFBPs) modulate IGF actions, we evaluated whether the increased IGF responsiveness might be associated with decreased inhibitory IGFBP-4 peptide levels. Rat vertebral cells were cultured for up to 20 days with or without dexamethasone (Dex). Cell layer proteins were extracted at day 6, 8, 14, and 20, conditioned media (CM) collected at day 8, 14, and 20, and total RNA isolated at day 14 and 20 of culture. Western blotting showed that cell layer IGFBP-4 levels were lower, while IGFBP-4 protease activity in CM was higher, in Dex-treated cultures. Addition of pregnancy-associated plasma protein-A (PAPP-A) antibody to CM abrogated IGFBP-4 proteolysis. PAPP-A mRNA levels were the same in control and Dex-treated cultures as evaluated by RT-PCR. Our data demonstrate that activity of the IGFBP-4 protease, PAPP-A, in rat bone cell cultures is increased by Dex via post-transcriptional mechanisms. Since IGFBP-4 mRNA levels in Dex-treated cultures were the same as in controls at day 8, slightly lower than in controls at day 14, and higher than in controls at day 20 as shown previously, the decreased IGFBP-4 peptide levels in Dex-treated cultures likely result from increased IGFBP-4 proteolysis by the elevated PAPP-A enzymatic activity. Our findings underscore a novel mechanism whereby GCs increase IGF responses in rat bone cells via PAPP-A-induced IGFBP-4 proteolysis.

Impact of circulating bone-resorbing cytokines on the subsequent bone loss following bone marrow transplantation

Cytokines including IL-6 and TNF-alpha play an important role in the pathogenesis of postmenopausal osteoporosis. However, the relationship between changes in the cytokine levels and subsequent bone loss in patients undergoing a bone marrow transplantation (BMT) is unclear. A total of 46 patients undergoing an allogeneic BMT were prospectively investigated. The bone turnover markers and the serum cytokines were measured before BMT and serially after BMT. Bone mineral density (BMD) was measured before and 1 year after BMT. At 1 year after BMT, the lumbar spine BMD had decreased by 4.8%, and the total proximal femoral BMD had decreased by 12.3%. The serum IL-6 and TNF-alpha levels increased until 2 and 3 weeks after BMT, respectively. The lumbar BMD was significantly decreased as the serum IL-6 and TNF-alpha levels increased by post-BMT 3 weeks. The lumbar BMD decreased significantly as the cumulative prednisolone and cyclosporine dose increased. Patients with GVHD > or =grade II had higher lumbar bone loss than patients with GVHD <grade I. In conclusion, immunosuppressants, GVHD occurrence and increase in bone-resorbing cytokines in the early post-BMT period were associated with later bone loss after BMT. Further studies are needed to elucidate the precise mechanism.

Transgenic expression of 11beta-hydroxysteroid dehydrogenase type 2 in osteoblasts reveals an anabolic role for endogenous glucocorticoids in bone

Glucocorticoid excess leads to bone loss, primarily by decreasing bone formation. However, a variety of in vitro models show that glucocorticoids can promote osteogenesis. To elucidate the role of endogenous glucocorticoids in bone metabolism, we developed transgenic (TG) mice in which a 2.3-kb Col1a1 promoter fragment drives 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) expression in mature osteoblasts. 11beta-HSD2 should metabolically inactivate endogenous glucocorticoids in the targeted cells, thereby reducing glucocorticoid signaling. The inhibitory effect of 300 nm hydrocortisone on percent collagen synthesis was blunted in TG calvariae, demonstrating that the transgene was active. Collagen synthesis rates were lower in TG calvarial organ cultures compared with wild-type. Trabecular bone parameters measured by microcomputed tomography were reduced in L3 vertebrae, but not femurs, of 7- and 24-wk-old TG females. These changes were also not seen in males. In addition, histomorphometry showed that osteoid surface was increased in TG female vertebrae, suggesting that mineralization may be impaired. Our data demonstrate that endogenous glucocorticoid signaling is required for normal vertebral trabecular bone volume and architecture in female mice.

The molecular actions of thyroid hormone in bone

Thyroid hormone (T(3)) is essential for the normal development of endochondral and intramembranous bone and plays an important role in the linear growth and maintenance of bone mass. Childhood T(3) deficiency results in retardation of skeletal development and growth arrest, whereas T(3) excess leads to accelerated growth and bone formation. In adult thyrotoxicosis, there is increased bone remodelling, characterized by an imbalance between bone resorption and formation, which results in net bone loss and an increased risk for posteoporotic fracture. These clinical observations demonstrate the importance of T(3) in skeletal development and metabolism. Nevertheless, the molecular mechanisms of T(3) action in bone are poorly understood. Here, we provide an overview of T(3) regulation of chondrocytes, osteoblasts and osteoclasts, and the actions of thyroid hormone receptor (TR) isoforms in skeletal development. The possible roles of T(3) and TRs in nuclear receptor crosstalk, prereceptor ligand metabolism, heparan sulfate proteoglycan synthesis and angiogenesis are also considered.

Production of insulin-like growth factor binding-proteins by bovine adrenomedullary cells: differential regulation by IGF-I and dexamethasone

In the present study we examined the production of insulin-like growth factor binding proteins (IGFBPs), in chromaffin cells, a model system for sympathetic neurons. Four IGFBPs of approximately 27, approximately 31, approximately 36 and a doublet of approximately 45-50 kDa, detected in Western ligand blots of conditioned medium, were identified in Western immunoblots as IGFBP-4, IGFBP-5, IGFBP-2 and IGFBP-3, respectively. In ligand blots IGFBP-3 and IGFBP-4 appeared as the most prominent species. IGF-I (1 nM) enhanced release of IGFBP-3 while dexamethasone (1 nM) diminished release of IGFBP-4. No significant proteolytic degradation of the IGFBPs was demonstrated. Cycloheximide completely attenuated release of the IGFBPs, indicating dependency on new synthesis of the proteins. These findings are consistent with autocrine modulation of the IGF system in bovine adrenomedullary chromaffin cells by IGFBPs. Furthermore, the specific stimulatory and inhibitory effects of IGF-I and dexamethasone, respectively, on release of the predominant species of IGFBP-3 and IGFBP-4, suggested that IGFBP production may be selectively modulated in a positive and negative manner.