PubMed Health⌕ Search

Biomedical subjects

A Horner

Publications and source records attributed to A Horner.

At least 19 recordsLinked to original sources
PubMed2006

Microarray analysis of healing rat Achilles tendon: evidence for glutamate signaling mechanisms and embryonic gene expression in healing tendon tissue.

Tendon healing is a complex process consisting of a large number of intricate pathways roughly divided into the phases of inflammation, proliferation, and remodeling. Although these processes have been extensively studied at a variety of levels in recent years, there is still much that remains unknown. This study used microarray analyses to investigate the process at a genetic level in healing rat Achilles tendon at 1, 7, and 21 days postinjury, roughly representing the inflammation, proliferation, and remodeling phases. An interesting temporal expression profile was demonstrated, identifying both known and novel genes and pathways involved in the progression of tendon healing. Both inflammatory response and pro-proliferative genes were shown to be significantly upregulated from 24 h postinjury through to 21 days. Day 7 showed the largest increase in genetic activity, particularly with the expression of collagens and other extracellular matrix genes. Interestingly, there was also evidence of central nervous system-like glutamate-based signaling machinery present in tendon cells, as has recently been shown in bone. This type of signaling mechanism has not previously been shown to exist in tendon. Another novel finding from these analyses is that there appears to be several genes upregulated during healing which have exclusively or primarily been characterized as key modulators of proliferation and patterning during embryonic development. This may suggest that similar pathways are employed in wound healing as in the tightly regulated progression of growth and development in the embryo. These results could be of use in designing novel gene-based therapies to increase the efficacy and efficiency of tendon healing.

Achilles Tendon↗
PubMed2002

Fibroblast growth factor signaling regulates Dach1 expression during skeletal development.

Dach1 is a mouse homologue of the Drosophila dachshund gene, which is a key regulator of cell fate determination during eye, leg, and brain development in the fly. We have investigated the expression and growth factor regulation of Dach1 during pre- and postnatal skeletal development in the mouse limb to understand better the function of Dach1. Dach1 was expressed in the distal mesenchyme of the early embryonic mouse limb bud and subsequently became restricted to the tips of digital cartilages. Dach1 protein was localized to postmitotic, prehypertrophic, and early hypertrophic chondrocytes during the initiation of ossification centers, but Dach1 was not expressed in growth plates that exhibited extensive ossification. Dach1 colocalized with Runx2/Cbfa1 in chondrocytes but not in the forming bone collar or primary spongiosa. Dach1 also colocalized with cyclin-dependent kinase inhibitors p27 (Kip1) and p57 (Kip2) in chondrocytes of the growth plate and in the epiphysis before the formation of the secondary ossification center. Because fibroblast growth factors (FGF), bone morphogenetic proteins (BMP), and hedgehog molecules (Hh) regulate skeletal patterning of the limb bud and chondrocyte maturation in developing endochondral bones, we investigated the regulation of Dach1 by these growth and differentiation factors. Expression of Dach1 in 11 days postcoitus mouse limb buds in organ culture was up-regulated by implanting beads soaked in FGF1, 2, 8, or 9 but not FGF10. BMP4-soaked beads down-regulated Dach1 expression, whereas Shh and bovine serum albumin had no effect. Furthermore, FGF4 or 8 could substitute for the apical ectodermal ridge in maintaining Dach1 expression in the limb buds. Immunolocalization of FGFR2 and FGFR3 revealed overlap with Dach1 expression during skeletal patterning and chondrocyte maturation. We conclude that Dach1 is a target gene of FGF signaling during limb skeletal development, and Dach1 may function as an intermediary in the FGF signaling pathway regulating cell proliferation or differentiation.

Animals↗
PubMed2001

Tie2 ligands angiopoietin-1 and angiopoietin-2 are coexpressed with vascular endothelial cell growth factor in growing human bone.

Angiogenesis is essential for bone growth and repair. Recent studies have shown that the endothelial-specific mitogen vascular endothelial growth factor (VEGF) is a key regulator of vascular invasion into the growth plate in infant and adolescent animals. In order to identify mechanisms regulating VEGF-induced angiogenesis in growing bone, we have investigated the expression of the angiopoietins (Ang-1 and Ang-2) in human neonatal ribs. Ang-1 and Ang-2 exhibited similar patterns of staining in the growing rib. In the cartilage, expression of Ang-1 and Ang-2 increased with chondrocyte maturation. Ang-1, Ang-2, and VEGF were not detected in the resting zone except adjacent to vascular canals, and maximum expression was detected at the cartilage bone interface. In the cartilage, Ang-2 was more highly expressed than Ang-1 or VEGF, with staining observed in the proliferating, hypertrophic, and mineralized zones. In the bone, Ang-1, Ang-2, and VEGF were detected in modeling and remodeling sites. Ang-1 was detected in the majority of osteoblasts, osteoclasts, and in some marrow space cells. Ang-2 was expressed at variable levels by osteoblasts and osteoclasts in modeling and remodeling bone. VEGF was detected in cells at bone surfaces and in the marrow spaces. Strong staining for VEGF was observed in osteoblasts and osteoclasts in modeling and remodeling bone. In the perichondrium, Ang-1, Ang-2, and VEGF were most highly expressed adjacent to the hypertrophic zone and at sites of bone collar formation. In the periosteum, Ang-1, Ang-2, and VEGF expression colocalized with alkaline phosphatase expression. These observations provide the first evidence for the expression of the angiopoietins in growing human bone in vivo. The distribution of Ang-1, Ang-2, and VEGF indicate these factors may play key roles in the regulation of angiogenesis at sites of endochondral ossification, intramembranous ossification, and bone turnover in the growing human skeleton.

Angiopoietin-1↗
PubMed2001

Mechanisms by which high-dose estrogen therapy produces anabolic skeletal effects in postmenopausal women: role of locally produced growth factors.

Conventional hormone replacement therapy acts primarily by preserving bone, but cannot restore lost bone in women with established osteoporosis. Studies in rodents have shown that high doses of estrogens have anabolic skeletal effects, and recent observations in a group of women treated long term with high doses of estrogen indicated that similar effects occur in humans. This study examines the hypothesis that locally produced growth factors, including transforming growth factor-beta (TGF-beta) and platelet-derived growth factors (PDGFs), are involved in mediating the anabolic effects of high-dose estrogen. Transiliac-crest bone biopsies were taken from ten women, aged 52-67 years (mean 58 years), who had been treated with high-dose estrogen for 15 years. Control samples were obtained from four age-matched postmenopausal women not receiving estrogen therapy. TGF-betas and PDGFs were analyzed for mRNA and protein expression by reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemistry. Results showed both TGF-beta1 and -beta2 mRNA, expressed as a ratio to GAPDH, were increased in the estrogen-treated group with an eightfold increase for TGF-beta1 (0.258 +/- 0.246 [mean +/- SD] vs. 0.032 +/- 0.053 in the control group, p = 0.02) and a twofold increase for TGF-beta2 (p = n.s.). TGF-beta3 analysis showed only negligible amounts in both groups. Protein expression levels for TGF-beta1, -beta2, -betaRI and -RII were higher in the estrogen-treated group than in controls, the most marked effects being seen for TGF-beta1. PDGF-A protein expression was also significantly higher in osteoblasts and osteocytes in women treated with estrogen, whereas PDGF-B showed only modest differences. The percentage of bone surface occupied by osteoclasts, as determined by tartrate-resistant acid phosphatase (TRAP) staining, was significantly reduced in the estrogen-treated group (p = 0.001). These results demonstrate that high-dose estrogen therapy is associated with increased TGF-beta, TGF-betaR, and PDGF synthesis and decreased osteoclast activity, consistent with the hypothesis that these growth factors may mediate the actions of estrogen in bone.

Acid Phosphatase↗
PubMed2001

The prevalence of environmental exposure to perceived asthma triggers in children with mild-to-moderate asthma: data from the Childhood Asthma Management Program (CAMP).

BACKGROUND: The Childhood Asthma Management Program, a 5-year randomized clinical trial of treatments for childhood asthma, has enrolled and characterized a cohort of 1041 children with mild-to-moderate asthma. OBJECTIVE: We sought to describe self-reported sensitivities and environmental exposures and investigate the relationships between self-report of these exposures as asthma triggers and their prevalence in the home. METHODS: Self-reports of sensitivities and home exposures were obtained by interview with the child or parent. Sensitivities were further assessed by using allergy skin testing (prick or puncture) against a core battery of allergens. Home exposures were further assessed by using analysis of a home dust sample. RESULTS: Environmental exposures were surprisingly common despite self-reported sensitivities to environmental factors. Of patients reporting that cigarette smoking frequently causes asthma symptoms, 26% reported having at least one parent who smokes cigarettes. Thirty-nine percent of patients reporting that exposure to animals frequently causes asthma symptoms live with a furry pet in their home. We found a smaller proportion of homes with a high level of cat allergen (P <.001) among the children who reported that animals frequently or always trigger asthma symptoms compared with those who reported that animals never or occasionally trigger asthma symptoms, suggesting modification of the home environment. No such results were seen for dog exposure. However, clinical symptoms did not reduce exposure to parental cigarette smoking (P =.15), house dust (P =.31), or damp and musty areas (P =.51). CONCLUSION: These data suggest that children with mild-to-moderate asthma are frequently symptomatic and exposed to a wide variety of environmental exposures that are perceived to trigger symptoms by means of self-report. Although environmental modification of asthmatic homes may occur, many children remain exposed to agents that are known to trigger their asthma.

Asthma↗
PubMed2001

Estrogen receptors alpha and beta are differentially expressed in developing human bone.

Estrogen plays an essential role in the development and maintenance of the skeleton; its effects are mediated via interactions with two estrogen receptor (ER) subtypes, alpha and beta. The aim of this study was to establish the cellular distribution of ERalpha and ERbeta in neonatal human rib bone. ERalpha and ERbeta immunoreactivity was seen in proliferative and prehypertrophic chondrocytes in the growth plate, with lower levels of expression in the late hypertrophic zone. Different patterns of expression of the two ERs were seen in bone. In cortical bone, intense staining for ERalpha was observed in osteoblasts and osteocytes adjacent to the periosteal-forming surface and in osteoclasts on the opposing resorbing surface. In cancellous bone, ERbeta was strongly expressed in both osteoblasts and osteocytes, whereas only low expression of ERalpha was seen in these areas. Nuclear and cytoplasmic staining for ERbeta was apparent in osteoclasts. These observations demonstrate distinct patterns of expression for the two ER subtypes in developing human bone and indicate functions in both the growth plate and mineralized bone. In the latter, ERalpha is predominantly expressed in cortical bone, whereas ERbeta shows higher levels of expression in cancellous bone.

Bone Development↗
PubMed2001

Colocalization of glucocorticoid and mineralocorticoid receptors in human bone.

Osteoporosis is a poorly understood but common complication of glucocorticoid therapy. The actions of glucocorticoids are mediated via glucocorticoid receptors (GRs), but in vitro, glucocorticoids also can bind to mineralocorticoid receptors (MRs). It is not known if MR protein is present in human bone and little is known of GR isoform expression (GRalpha and GRbeta). GR and MR protein expression and possible sites of action were investigated in neonatal rib and adult iliac crest biopsy specimens using antibodies specific for MR, GRalpha, and GRalphabeta. Colocalization [MR GRalpha] [MR GRalphabeta] was performed using fluorescent-conjugated secondary antibodies. GRalpha, GRbeta, and MR show distinct but overlapping patterns of expression, suggesting important functions for each receptor type. Osteoclasts showed no staining for GRalpha but strong staining for GRalphabeta, indicating expression of GRbeta and a specific role in addition to antagonizing the transcriptional activity of GRalpha. MR also was observed in osteoclasts and colocalized with GRalphabeta. Coexpression of MR, GRalpha, and GRalphabeta was seen in osteoblasts. Reverse-transcription-polymerase chain reaction (RT-PCR) of cultured osteoblast RNA confirmed expression of both GRalpha and GRbeta. Osteocytes stained with MR, GRalpha, and GRalphabeta antibodies but to a lesser degree than osteoblasts. In the neonatal rib cartilage, staining for GRalpha, GRalphabeta, and MR was present in approximately one-half of the resting and hypertrophic chondrocytes and in most of proliferating chondrocytes and chondrocytes within the mineralizing matrix. Identification of MR raises the possibility that the physiological and pharmacologic effects of glucocorticoids on bone may be mediated via MR as well as GR and that GRalpha, GRbeta, and MR synergize to influence corticosteroid metabolism in human bone.

Bone and Bones↗
PubMed2000

Megakaryocyte population in human bone marrow increases with estrogen treatment: a role in bone remodeling?

Skeletal effects of conventional hormone replacement therapy (HRT) are predominately antiresorptive, while high doses of estrogen have anabolic effects. The mechanisms mediating these effects are unclear but may involve cells in the bone marrow. We have investigated the in vivo effects of estrogen on the megakaryocyte (MK) population in bone marrow in 10 postmenopausal women before and after 2 years of conventional HRT, in 11 women after long-term, high-dose estradiol therapy, and in 2 premenopausal and 4 postmenopausal women who had received no previous estrogen treatment. Transiliac crest biopsies were halved and either decalcified and paraffin wax embedded for immunolocalization studies or dehydrated and embedded in LR White resin for histology. MKs were identified morphologically, and the bone marrow cell population and MK number quantified by cell counting in a defined area of view (1 mm(2)) from 5 randomly selected fields of bone marrow. Compared with pretreatment values, significantly higher MK numbers were found after conventional HRT treatment (before treatment, mean +/- SEM; 7.3 +/- 1.1 vs. after treatment, 18.0 +/- 1.6/5 mm(2); p < 0.0001), while the greatest MK number was associated with long-term, high-dose estradiol treatment (32.8 +/- 2.1/5 mm(2); p < 0.0001). Total bone marrow cell number did not differ significantly between groups. Immunolocalization studies revealed more intense estrogen receptor (ER)beta expression in MKs in the high-dose estradiol-treated group but similar levels of weak ERalpha staining in MKs in the control and high-dose estrogen-treated groups. Positive immunoreactivity for transforming growth factor (TGF)beta1, 2, and 3 and TGFbeta receptor I, II, and III was detected in MKs, with more intense staining being demonstrated in the high-dose estradiol-treated group, particularly for TGFbeta2 and TGFbetaRI and II. Our results demonstrate an increase in the MK population in bone marrow from women treated with estrogen. The ability of MKs to express ERs and synthesise TGFbeta, a potent mitogen in osteoblast differentiation, suggests that these cells may play a role in mediating estrogen-induced effects on bone.

Adult↗
PubMed2000

The localization of thyroid hormone receptor mRNAs in human bone.

Thyroid hormones have well-documented effects on the skeleton although the mechanism of their action on bone is poorly understood. We have recently reported the presence of different thyroid hormone receptor isoforms in human bone. However, there is evidence to suggest that the expression of thyroid hormone receptor (TR) protein may not necessarily correlate with its mRNA. In this study, we used specific digoxigenin-labeled ribo probes to investigate the expression of TRalpha1, variant TRalpha2, TRbeta1, and in particular TRbeta2 mRNA in human osteophytic bone and osteoclastoma tissue in situ. The number of positive cells was expressed as the percentage of the total number of cells of the same phenotype. In osteophytes, at sites of endochondral ossification, TRalpha1, variant TRalpha2, TRbeta1, and TRbeta2 mRNA were widely distributed in undifferentiated, proliferating, mature and hypertrophic chondrocytes. At sites of bone remodeling, TRalpha1 mRNA was expressed in the majority (> 90%) of osteoblasts. TRbeta1 and the variant TR-alpha2 mRNA were moderately expressed in approximately 75% of cells with only a few osteoblasts (< 25%) expressing TRbeta2 mRNA. All the TR transcripts were highly expressed in multinucleated osteoclasts in osteoclastoma tissue. The distribution of TR mRNAs was similar to TR receptor protein expression (as we have previously reported) in both osteophytic bone and osteoclastoma tissue except TRalpha1 mRNA that was highly expressed in osteoclasts and in undifferentiated, proliferating, mature, and hypertrophic chondrocytes in contrast to its receptor protein expression. This study highlights the importance of studying both TR mRNA and receptor proteins in triiodothyronine (T3) responsive tissues. This is also the first demonstration of the presence of TRbeta2 mRNA in bone. The role of TRbeta2 in mediating the actions of thyroid hormones in bone is not known and requires further investigation.

Aged↗
PubMed2000

The localization of the functional glucocorticoid receptor alpha in human bone.

Glucocorticoids have well-documented effects on the skeleton, although their mechanism of action is still poorly understood. The actions of glucocorticoids on bone cells are mediated, in part, directly via specific receptors. The presence of these receptors has been demonstrated in both rodent and human osteoblastic cells in vitro, but their presence in human bone in vivo has not been reported. In this study, we have used specific affinity purified polyclonal antibodies to the functional glucocorticoid receptor alpha (GRalpha) to investigate its expression in both developing and adult human bone using sections of neonatal rib, calvarial, and vertebral bones, tibial growth plates from adolescents, and iliac crest biopsies from adults who were to undergo liver transplantation. In the tibial growth plates, GRalpha was predominantly expressed in the hypertrophic chondrocytes within the cartilage. In the primary spongiosa, the receptor was highly expressed by osteoblasts at sites of bone modeling. Within the bone marrow, receptors were also detected in mononuclear cells and in endothelial cells of blood vessels. In the neonatal rib and vertebrae, GRalpha was widely distributed at sites of endochondral bone formation in resting, proliferating, mature, and hypertrophic chondrocytes. They were also highly expressed in osteoblasts at sites of bone modeling. At sites of intramembranous ossification in neonatal calvarial bone and rib periosteum, GRa was widely expressed in cells within the fibrous tissue and in osteoblasts at both the bone-forming surface and at modeling sites. In the iliac crests from adults, GRalpha was predominantly expressed in osteocytes. The receptors were not detected in osteoclasts. Our results show for the first time the presence of the functional GRalpha in human bone in situ and suggest that the actions of glucocorticoids on bone may be mediated, in part, directly via the GR at different stages of life. The absence of receptor expression in osteoclasts also suggests that the effects of glucocorticoids on bone resorption may be mediated indirectly.

Adolescent↗
PubMed1999

Tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) distribution in normal and pathological human bone.

Degradation of skeletal connective tissue is regulated, at least in part, by the balance between matrix metalloproteinases (MMPs) and tissue inhibitors of matrix metalloproteinase (TIMPs), their natural inhibitors. The balance between MMPs and TIMPs may therefore be a determinant of normal bone turnover, and imbalance could thus lead to reduced organization of bone structure. To test this hypothesis, the cellular expression of MMPs and TIMP-1 was investigated by immunohistochemistry in human neonatal rib and osteophytic and heterotopic bone; these differ in their structure, with heterotopic bone showing the least and normal rib the most organized development. In all samples, high levels of MMPs were expressed. Collagenase and stromelysin-2 were detected in chondrocytes, osteoblasts, and osteoclasts, whereas gelatinase-B was confined to osteoclasts and mononuclear cells. Matrix-associated stromelysin-1 was present in fibrous tissue and osteoid. In contrast, the expression of TIMP-1 varied markedly between the three types of bone. In heterotopic bone only occasional low level TIMP-1 expression was detected in chondrocytes and osteoblasts. Osteophytic bone showed varying levels of TIMP-1, which was matrix-bound in fibrous tissue and cell-associated in osteoblasts, chondrocytes, and occasional mononuclear cells. In both types of bone, expression of TIMP-1 by osteoclasts was absent despite large numbers of these cells. Neonatal rib bone showed consistent expression of TIMP-1, particularly in chondrocytes, osteoblasts, and lining cells. In contrast to pathological bone, many osteoclasts were TIMP-1 positive. These results suggest that, in heterotopic and osteophytic bone, the low levels of TIMP-1, and in particular its absence in osteoclasts, may partly explain the more poorly organized bone formation in these pathological bone samples. Furthermore, TIMP-1 may play a role in the regulation of bone modeling and remodeling in normal developing human bone.

Adult↗
PubMed1999

Immunolocalisation of vascular endothelial growth factor (VEGF) in human neonatal growth plate cartilage.

Angiogenesis is essential for the replacement of cartilage by bone during growth and repair. In order to obtain a better understanding of the mechanisms regulating vascular invasion at sites of endochondral ossification we have investigated the expression of the endothelial cell-specific mitogen, vascular endothelial growth factor (VEGF), by chondrocytes in human neonatal growth plates. VEGF was absent from chondrocytes in the resting zone and only weakly expressed by occasional chondrocytes in the proliferating region. In the hypertrophic zone the number of chondrocytes stained and the intensity of staining for VEGF increased with chondrocyte hypertrophy, maximum expression of VEGF being observed in chondrocytes in the lower hypertrophic and mineralised regions of the cartilage. These observations provide the first demonstration of the presence of VEGF in situ in developing human bone and are consistent with in vitro observations demonstrating the upregulation of proangiogenic growth factor production with increasing chondrocyte hypertrophy. The presence of numerous small blood vessels and vascular structures in the subchondral region where VEGF expression was maximal indicates that VEGF produced by hypertrophic chondrocytes may play a key role in the regulation of vascular invasion of the growth plate.

Cartilage↗
PubMed1998

Stromelysin-1 (MMP-3) and stromelysin-2 (MMP-10) expression in developing human bone: potential roles in skeletal development.

Stromelysin, a member of the matrix metalloproteinase family, demonstrates wide substrate specificity with the ability to degrade proteoglycan, fibronectin, laminin, casein, and the nonhelical region of collagen. The two forms of stromelysin (SL), types 1 (MMP-3) and 2 (MMP-10), share 82% sequence homology, but exhibit differences in cellular synthesis and inducibility by cytokines and growth factors in vitro. However, the distribution of the two isoforms in bone has not been reported. We investigated the presence of SL-1 and SL-2 in human osteophytic and neonatal rib bone using immunohistochemistry and, combined with a new method of in situ zymography, determined the activity of the immunolocalized stromelysins. Latent SL-1 was strongly expressed in the extracellular matrix in fibrous tissue surrounding areas of endochondral ossification in osteophytes, and adjacent to the periosteum of fetal rib bone. Active SL-1 expression was detected in osteocytes and the matrix surrounding osteocytic lacunae. SL-2 showed intense cell-associated staining at sites of resorption in areas of endochondral ossification and in resorptive cells at the chondro-osseous junction, which correlated with enzyme activity detected by zymography. Within the rib, active SL-2 expression was localized in chondrocytes of the growth plate, whereas only occasional SL-1 signal was evident. Vascular areas showed strong SL-2 staining with some proteolytic activity. SL-2, but not SL-1, was strongly expressed in osteoclasts and most mononuclear cells within the marrow. At sites of bone formation both isoforms were expressed by osteoblasts with SL-1 also present in osteoid. These results demonstrate, for the first time, the differential expression of SL-1 and SL-2 in developing human bone, indicating specific roles for the two isoforms. In situ zymography demonstrates that SL-2 is produced in an active form with associated degradation, whereas SL-1, in a matrix-bound proenzyme form, may act as a reservoir for later activation.

Glycoproteins↗
PubMed1998

Expression and distribution of transforming growth factor-beta isoforms and their signaling receptors in growing human bone.

Transforming growth factors type beta (TGF-beta1, -beta2, and -beta3) are potent stimulators of bone formation and have been shown to regulate chondrocyte, osteoblast, and osteoclast formation and function. However, the distribution of the different isoforms and their signaling receptors in human bone in vivo has not previously been reported. Using samples of normal (neonatal rib) and pathological (osteophytic) developing human bone, we have investigated the expression of the different TGF-beta isoforms and their signaling receptors (TGF-betaRI and RII) at the messenger ribonucleic acid (mRNA) and protein levels by in situ hybridization and immunolocalization to establish the sites of TGF-beta production and their possible sites of action during human bone development in vivo. All three TGF-beta isoforms and the receptors were detected at sites of endochondral and intramembranous ossification. At sites of endochondral ossification, TGF-beta2 was detected in all zones of the cartilage, with the highest expression seen in the hypertrophic and mineralizing zones. TGF-beta3 was detected in proliferative and hypertrophic zone chondrocytes, while TGF-beta1 expression was restricted to the proliferative and upper hypertrophic zones. TGF-betaRI and RII exhibited similar distributions with maximum expression in the hypertrophic and mineralizing zones in the neonatal rib but in the resting/proliferative zone in the developing osteophyte. At sites of intramembranous ossification TGF-beta3 was the most widely distributed isoform and showed both matrix- and cell-associated staining. TGF-beta2 and -beta1 were expressed almost exclusively at sites of mineralization. These observations demonstrate that the different TGF-beta isoforms and their receptors exhibit distinct but overlapping patterns of expression, and support the hypothesis that they are involved in the regulation of endochondral and intramembranous ossification during human bone development in vivo.

Activin Receptors, Type I↗
PubMed1997

The expression of thyroid hormone receptors in human bone.

The mechanism of action of thyroid hormones on bone is poorly understood. Thyroid hormones may act on bone cells either indirectly by increasing secretion of growth hormone (GH) and insulin-like growth factor-1 (IGF-1), or directly by influencing target genes via specific nuclear receptors. The presence of thyroid hormone receptors (TRs) has been demonstrated in human and rodent osteoblast-like cells and cell lines and recently in osteoclasts derived from an osteoclastoma in vitro. However, their presence in human bone in situ has not been reported. We have used specific polyclonal antibodies to TR-alpha 1, -alpha 2, and -beta 1 to investigate the expression of these receptors in sections of human osteophytes and heterotopic bone. Osteoblasts and osteoclasts were identified by alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP), respectively, whereas chondrocytes were identified morphologically. At sites of endochondral and intramembranous bone formation, TR-beta 1 and the splice variant -alpha 2 were widely expressed by proliferating, mature, and hypertrophic chondrocytes and also in cells within the fibrous tissue and at the bone forming surfaces, respectively. They were also detected in osteoblasts, osteoclasts, and a few osteocytes at sites of bone remodeling. In contrast, TR-alpha 1 was the least expressed and was present mainly in osteoblasts at remodeling sites and in a few mature and undifferentiated chondrocytes. Our results show, for the first time, the presence and distribution of TRs in human bone in situ and suggest that the skeletal actions of thyroid hormones may be mediated via these receptors. Further studies are required to define the role of the individual receptor isoforms in bone metabolism.

Acid Phosphatase↗
PubMed1997

Distribution of matrix metalloproteinases and their inhibitor, TIMP-1, in developing human osteophytic bone.

Connective tissues synthesise and secrete a family of matrix metalloproteinases (MMPs) which are capable of degrading most components of the extracellular matrix. Animal studies suggest that the MMPs play a role in bone turnover. Using specific polyclonal antisera, immunohistochemistry was used to determine the patterns of synthesis and distribution of collagenase (MMP-1), stromelysin (MMP-3), gelatinase A (MMP-2) and gelatinase B (MMP-9) and of the tissue inhibitor of metalloproteinases-1 (TIMP-1) within developing human osteophytic bone. The different MMPs and TIMP showed distinct patterns of localisation. Collagenase expression was seen at sites of vascular invasion, in osteoblasts synthesising new matrix and in some osteoclasts at sites of resorption. Chondrocytes demonstrated variable levels of collagenase and stromelysin expression throughout the proliferative and hypertrophic regions, stromelysin showing both cell-associated and strong matrix staining. Intense gelatinase B expression was observed at sites of bone resorption in osteoclasts and mononuclear cells. Gelatinase A was only weakly expressed in the fibrocartilage adjacent to areas of endochondral ossification. There was widespread but variable expression of TIMP-1 throughout the fibrous tissue, cartilage and bone. These results indicate that MMPs play a role in the development of human bone from cartilage and fibrous tissue and are likely to have multiple functions.

Bone and Bones↗
PubMed1997

The localization of androgen receptors in human bone.

Androgens have important effects on the human skeleton, and deficiency has been associated with bone loss in both males and females. The skeletal actions of androgens may be mediated directly via the androgen receptor (AR) or indirectly via the estrogen receptor after aromatization to estrogens. The presence of androgen receptors has been demonstrated in bone cells and chondrocytes in vitro, but their presence in human bone in situ has not been reported. In order to provide further evidence for a direct action of androgens on bone via androgen receptors, we have used specific monoclonal antibodies to investigate the expression of human AR in normal developing and osteophytic bone of both sexes. In the growth plates from the developing bone, androgen receptors were predominantly expressed in hypertrophic chondrocytes and in osteoblasts at sites of bone formation. They were also observed in osteocytes in the bone, and in mononuclear cells and endothelial cells of blood vessels within the bone marrow. In the osteophytes, androgen receptors were widely distributed at sites of endochondral ossification in proliferating, mature, and hypertrophic chondrocytes and at sites of bone remodeling in osteoblasts. They were also expressed in osteocytes and mononuclear cells within the bone marrow. The pattern and number of cells expressing the receptor was similar in both sexes. Our results show for the first time the presence and distribution of androgen receptors in normal developing human and osteophytic bone in situ and further provide evidence for a direct action of androgens on bone and cartilage cells.

Adenocarcinoma↗