Regulation of bone sialoprotein mRNA by steroid hormones

Osteoimmunomodulation, osseointegration, and in vivo mechanical integrity of pure Mg coated with HA nanorod/pore-sealed MgO bilayer

Fast degradation of Mg-based implants results in the loss of mechanical integrity and poor osseointegration. Herein, a bilayer-structured coating (termed as HAT), comprising an outer layer of hydroxyapatite (HA) nanorods and an inner layer of pores-sealed MgO with HA/Mg(OH)2, was formed on Mg using plasma electrolytic oxidation and hydrothermal treatment. Osteoimmunomodulation, osseointegration, mechanical integrity, and bone-implant interfacial structure evolution of the HAT-coated Mg were investigated by implantation in rabbit femora, together with Mg coated with plasma electrolytic oxidized porous MgO (termed as PEO0) and bare Mg. As compared to PEO0-coated and bare Mg, HAT-coated Mg greatly downregulated pro-inflammatory TNF-α and IL-1β, upregulated anti-inflammatory IL-10, and suppressed osteoclastogenesis, modulating the surrounding microenvironment toward favoring the recruitment of osteogenetic cells. Moreover, HAT-coated Mg accelerated bone sialoprotein and osteopontin secretion of osteogenetic cells and their mineralization to form a cement line matrix. It also promoted the differentiation of osteogenetic cells, secretion of collagen overlying on the cement line matrix, inducing an earlier and more pronounced bone matrix formation. The cement line matrix wrapped the HA nanorods and filled the interrod spaces of the HAT coating, forming strong interdigitation at the bone-coating interface, and therefore, yielding enhanced osseointegration by means of contact osteogenesis. Due to the considerably reduced corrosion of Mg by the pores-sealed bilayer structure of HAT coating, HAT-coated Mg maintained the mechanical integrity for a longer duration than PEO0-coated and bare Mg. It is clarified that the degradation of MgO and HA, rather than delamination, was the vanishing mode of PEO0 and HAT coatings during long-term implantation, avoiding osteolysis induced by the delamination-generated particles.

Osteopontin facilitates ultraviolet B-induced squamous cell carcinoma development

BACKGROUND

Osteopontin (OPN) is a matricellular glycoprotein that is markedly expressed in cutaneous squamous cell carcinomas (cSCCs) and in actinic keratoses implicating its role in photocarcinogenesis.

OBJECTIVE

To determine whether OPN facilitates the development of cSCC and its function.

METHODS

cSCCs development was compared between wild-type (WT) and OPN-null mice subjected to UVB irradiation for 43 weeks. UVB-induced OPN expression was determined by Western blot, immunoprecipitation, ELISA, and semi-quantitative RT-PCR. Epidermal layer and TUNEL analyses assessed if OPN mediates UVB-induced epidermal hyperplasia or suppresses UVB-induced apoptosis of basal keratinocytes, respectively. In vitro experiments determined whether OPN enhances cell survival of UVB-induced apoptosis and its potential mechanisms. Immunohistochemical analyses of epidermis assessed the expression of CD44 and focal adhesion kinase (FAK), molecules that mediate OPN survival function.

RESULTS

Compared to female WT mice, OPN-null mice did not develop cSCCs. UVB irradiation stimulated OPN protein expression in the dorsal skin by 11h and remains high at 24-48h. OPN did not mediate UVB-induced epidermal hyperplasia; instead, it protected basal keratinocytes from undergoing apoptosis upon UVB exposure. Likewise, the addition of OPN suppressed UVB-induced OPN-null cSCC cell apoptosis, the activation of caspase-9 activity, and increased phosphorylation of FAK at Y397. Furthermore, the expression of CD44 and FAK in WT mice epidermis was greater than that of OPN-null mice prior to and during early acute UVB exposure.

CONCLUSION

These data support the hypothesis that chronic UVB-induced OPN expression protects the survival of initiated basal keratinocytes and, consequently, facilitates cSCC develop.

Silencing diacylglycerol kinase-theta expression reduces steroid hormone biosynthesis and cholesterol metabolism in human adrenocortical cells

Diacylglycerol kinase theta (DGKθ) plays a pivotal role in regulating adrenocortical steroidogenesis by synthesizing the ligand for the nuclear receptor steroidogenic factor 1 (SF1). In response to activation of the cAMP signaling cascade nuclear DGK activity is rapidly increased, facilitating PA-mediated, SF1-dependent transcription of genes required for cortisol and dehydroepiandrosterone (DHEA) biosynthesis. Based on our previous work identifying DGKθ as the enzyme that produces the agonist for SF1, we generated a tetracycline-inducible H295R stable cell line to express a short hairpin RNA (shRNA) against DGKθ and characterized the effect of silencing DGKθ on adrenocortical gene expression. Genome-wide DNA microarray analysis revealed that silencing DGKθ expression alters the expression of multiple genes, including steroidogenic genes, nuclear receptors and genes involved in sphingolipid, phospholipid and cholesterol metabolism. Interestingly, the expression of sterol regulatory element binding proteins (SREBPs) was also suppressed. Consistent with the suppression of SREBPs, we observed a down-regulation of multiple SREBP target genes, including 3-hydroxy-3-methylglutary coenzyme A reductase (HMG-CoA red) and CYP51, concomitant with a decrease in cellular cholesterol. DGKθ knockdown cells exhibited a reduced capacity to metabolize PA, with a down-regulation of lipin and phospholipase D (PLD) isoforms. In contrast, suppression of DGKθ increased the expression of several genes in the sphingolipid metabolic pathway, including acid ceramidase (ASAH1) and sphingosine kinases (SPHK). In summary, these data demonstrate that DGKθ plays an important role in steroid hormone production in human adrenocortical cells.

Topographic scale-range synergy at the functional bone/implant interface

We sought to explore the biological mechanisms by which endosseous implant surface topography contributes to bone anchorage. To address this experimentally, we implanted five groups of custom-made commercially pure titanium implants of varying surface topographical complexity in rat femora for 9 days; subjected them to mechanical testing; and then examined the interfacial bone matrix by electron microscopy. The five implant surfaces were prepared by combinations of dual acid etching and grit blasting the titanium substrates and, in some cases, modifying the created surfaces with the deposition of nanocrystals of calcium phosphate, which resulted in 10 samples per group. In parallel, we cultured rat bone marrow cells on surrogate implants constructed from polymer resin coated with the same calcium phosphate nanocrystals, and monitored the deposition of bone sialoprotein by transmission electron immunohisto-micrography. We found that implant samples modified with sub-micron scale crystals were bone-bonding, as described by the interdigitation of a mineralized cement line matrix with the underlying implant surface. The in vitro assay showed that bone sialoprotein could be deposited in the interstices between, and undercuts below, the nanocrystals. In addition, when mineralized, the cement line matrix globules occupied micron-sized pits in the implant surfaces, and in part obliterated them, creating an additional form of anchorage. Our results also showed that collagen, elaborated by the osteogenic cells, wrapped around the coarse-micron features, and became mineralized in the normal course of bone formation. This provided a mechanism by which coarse-micron implant features contributed to a functional interface, which we have previously described, that is capable of resisting the mechanical loading that increases as peri-implant bone matures. Thus, our findings provide mechanistic explanations for the biologically-relevant criteria that can be employed to assess the importance of implant surface topography at different scale-ranges.

Review: The diabetic bone: a cellular and molecular perspective

With the increasing worldwide prevalence of diabetes the resulting complications, their consequences and treatment will lead to a greater social and financial burden on society. One of the many organs to be affected is bone. Loss of bone is observed in type 1 diabetes, in extreme cases mirroring osteoporosis, thus a greater risk of fracture. In the case of type 2 diabetes, both a loss and an increase of bone has been observed, although in both cases the quality of the bone overall was poorer, again leading to a greater risk of fracture. Once a fracture has occurred, healing is delayed in diabetes, including nonunion. The reasons leading to such changes in the state of the bone and fracture healing in diabetes is under investigation, including at the cellular and the molecular levels. In comparison with our knowledge of events in normal bone homeostasis and fracture healing, that for diabetes is much more limited, particularly in patients. However, progress is being made, especially with the use of animal models for both diabetes types. Identifying the molecular and cellular changes in the bone in diabetes and understanding how they arise will allow for targeted intervention to improve diabetic bone, thus helping to counter conditions such as Charcot foot as well as preventing fracture and accelerating healing when a fracture does occur.

Gene expression and distribution of key bone turnover markers in the callus of estrogen-deficient, vitamin D-depleted rats

An experimental rat model was used to test the hypothesis that in osteoporosis (OP) the molecular composition of the extracellular matrix in the fracture callus is disturbed. OP was induced at 10 weeks of age by ovariectomy and a vitamin D(3)-deficient diet, and sham-operated animals fed normal diet served as controls. Three months later a closed tibial fracture was made and stabilized with an intramedullary nail. After 3 and 6 weeks of healing, the animals were killed and the fracture calluses examined with global gene expression, in situ mRNA expression, and ultrastructural protein distribution of four bone turnover markers: osteopontin, bone sialoprotein, tartrate-resistant acid phosphatase, and cathepsin K. Global gene expression showed a relatively small number of differently regulated genes, mostly upregulated and at 3 weeks. The four chosen markers were not differently regulated, and only minor differences in the in situ mRNA expression and ultrastructural protein distribution were detected. Gene expression and composition of fracture calluses are not generally disturbed in experimental OP.

Ultrastructural immunolocalisation of bone sialoprotein in the osteocartilagenous interface of the equine third carpal bone

REASONS FOR PERFORMING STUDY

One of the most common causes of lameness in racehorses is osteoarthritis (OA). Pathogenesis is not clear and pathological processes of the different joint tissues interact in often progressive events. The interface between cartilage and newly synthesised bone has been shown to be particularly enriched in bone sialoprotein (BSP), a cell-binding matrix protein.

OBJECTIVES

To establish whether changes in the concentration of BSP may serve as a marker for early biochemical changes of the subchondral bone.

METHODS

Articular cartilage, cartilage/bone interface and subchondral bone of the proximal third carpal bone from 3 Standardbred trotters were analysed ultrastructurally for the presence of BSP in normal and degenerative areas.

RESULTS

A marked increase of BSP in the cartilage/bone interface with degenerative changes of the bone and cartilage compared to the morphologically intact cartilage/bone interface was noted, but levels of the protein were distinctly lower in the distal bone.

CONCLUSIONS

The results indicate that BSP has the potential to be used as a marker for changes in bone metabolism in the subchondral bone.

POTENTIAL RELEVANCE

Tools to monitor early biochemical changes within the connective tissues of the joint in vivo are essential in studies of the pathogenesis of OA. These could be used to monitor and understand such changes in relation to load, exercise, training programmes, inflammation and the development of OA.

A new method for the preparation of bioactive calcium phosphate films hybridized with 1alpha,25-dihydroxyvitamin D3

The primary goal of this investigation was to develop a calcium phosphate film hybridized with 1alpha,25-dihydroxyvitamin D(3) for the improvement of osteoconductivity of bone substitutes. The hybrid films (hCaP) were prepared at the different concentrations of 1 x 10(-10), 1 x 10(-8), and 1 x 10(-6) M designated as hCaPL, hCaPM, and hCaPH, respectively. The change of the hormone concentration during the preparation of the hybrid films did not cause significant variations on the physical properties of hCaPs, i.e. surface morphology and roughness. On the other hand, X-ray photon spectroscope (XPS) measurements revealed that the concentration change affected the chemical composition of the hybrid films. Recruitment of osteoblast-like MG-63 cells was considerably improved on hCaPs compared to tissue culture plate (TCP). However, cell proliferation on hCaPs was substantially suppressed and inversely proportional to the hormone concentration used. It was observed that bone-like nodules which consisted of bead-like components and well-developed matrix were rapidly formed on hCaPs. Masson's trichrome and safranin-O stainings elucidated that the bead-like components were MG-63 cells. Safranin-O staining showed that proteoglycan was produced actively. These results indicate that the cells cultured on hCaPs were strongly stimulated by the hormone to produce proteoglycan which can be considered as an induction of premature bone formation. The number of the nodules was increased with hormone concentration and most pronounced at the hCaPH. Gene expression patterns of alkaline phosphatase (ALP), transforming growth factor-beta (TGF-beta), and osteopontin (OPN) were strongly modulated by hybridized the hormone. For ALP and OPN, gene expressions were activated earlier on hCaPs than untreated calcium phosphate (CaP) confirming the effect of the hybridization was substantial. The TGF-beta gene expression was immediately activated after seeding but difference between samples was not significant suggesting that the gene expression was modulated not by the hormone hybridization but by CaP itself. As a result, hybridization of 1,25(OH)(2)D(3) with CaP can be a potentially strong candidate to promote osteoconductivity of implant materials.

Modulation of bone resorption by phosphorylation state of bone sialoprotein

We have determined transmembrane protein tyrosine phosphorylation (outside-in signaling) in cultured osteoclasts and macrophages in response to added native purified bone sialoprotein (nBSP) and its dephosphorylated form (dBSP). There were selective/differential and potent inhibitory effects by dBSP and minimal effect by nBSP on intracellular tyrosine phosphorylation in macrophages and osteoclasts. Further studies on the downstream gene expression effects led to identification of a large number of differentially expressed genes in response to nBSP relative to dBSP in both macrophages and osteoclasts. These studies were extended to a bone resorption model using live mouse neonatal calvarial bone organ cultures stimulated by parathyroid hormone (PTH) to undergo bone resorption. Inclusion of nBSP in such cultures showed no effect on type I collagen telopeptide fragment release, hence overall bone resorption, whereas addition of dBSP abolished the PTH-induced bone resorption. The inhibition of bone resorption by dBSP was shown to be unique since in complementary experiments use of integrin receptor binding ligand, GRGDS peptide, offered only partial reduction on overall bone resorption. Quantitative RANKL analysis indicated that mechanistically the PTH-induced bone resorption was inhibited by dBSP via down-regulation of the osteoblastic RANKL production. This conclusion was supported by the RANKL analysis in cultured MC3T3-E1 osteoblast cells. Overall, these studies provided direct evidence for the involvement of covalently bound phosphates on BSP in receptor mediated "outside-in" signaling via transmembrane tyrosine phosphorylation with concurrent effects on downstream gene expressions. The use of a live bone organ culture system augmented these results with further evidence that links the observed in vivo variable state of phosphorylation with bone remodeling.

Altered osteoclast development and function in osteopontin deficient mice

The role of osteopontin in bone resorption was elucidated by studies of mice with knock out of the osteopontin gene generated by a different approach compared to previous models. Thus, a targeting vector with the promoter region as well as exons 1, 2, and 3 of the osteopontin gene was replaced by a loxP-flanked Neo-TK cassette, and this cassette was eliminated through transient expression of Cre recombinase. The recombined ES cells were used to create mice lacking expression of the osteopontin gene. Tissues from these mice were subjected structural and molecular analyses including morphometry and proteomics. The bone of the null mice contained no osteopontin but showed no significant alterations with regard to other bone proteins. The bone volume was normal in young null animals but in the lower metaphysis, the volume and number of osteoclasts were increased. Notably, the volume and length of the osteoclast ruffled border was several folds lower, indicating a lower resorptive capacity. The null mice did not develop the bone loss characteristic for osteoporosis demonstrated in old wild-type female animals. This quantitative study demonstrates a bone phenotype in the osteopontin null mice of all ages. The data provides further evidence for a role of osteopontin in osteoclast activity.

Bone sialoprotein and its transcriptional regulatory mechanism

BACKGROUND AND OBJECTIVE

Bone sialoprotein is a mineralized tissue-specific noncollagenous protein that is glycosylated, phosphorylated and sulfated. The temporo-spatial deposition of bone sialoprotein into the extracellular matrix of bone, and the ability of bone sialoprotein to nucleate hydroxyapatite crystal formation, indicates a potential role for bone sialoprotein in the initial mineralization of bone, dentin and cementum. Bone sialoprotein is also expressed in breast, lung, thyroid and prostate cancers.

MATERIAL AND METHODS

We used osteoblast-like cells (rat osteosarcoma cell lines ROS17/2.8 and UMR106, rat stromal bone marrow RBMC-D8 cells and human osteosarcoma Saos2 cells), and breast and prostate cancer cells to investigate the transcriptional regulation of bone sialoprotein. To determine the molecular basis of the transcriptional regulation of the bone sialoprotein gene, we conducted northern hybridization, transient transfection analyses with chimeric constructs of the bone sialoprotein gene promoter linked to a luciferase reporter gene and gel mobility shift assays.

RESULTS

Bone sialoprotein transcription is regulated by hormones, growth factors and cytokines through tyrosine kinase, mitogen-activated protein kinase and cAMP-dependent pathways. Microcalcifications are often associated with human mammary lesions, particularly with breast carcinomas. Expression of bone sialoprotein by cancer cells could play a major role in the mineral deposition and in preferred bone homing of breast cancer cells.

CONCLUSION

Bone sialoprotein protects cells from complement-mediated cellular lysis, activates matrix metalloproteinase 2 and has an angiogenic capacity. Therefore, regulation of the bone sialoprotein gene is potentially important in the differentiation of osteoblasts, bone matrix mineralization and tumor metastasis. This review highlights the function and transcriptional regulation of bone sialoprotein.

Runx2- and histone deacetylase 3-mediated repression is relieved in differentiating human osteoblast cells to allow high bone sialoprotein expression

Bone sialoprotein (BSP) is a bone matrix glycoprotein whose expression coincides with terminal osteoblastic differentiation and the onset of mineralization. In this study we show that BSP expression is considerably increased in confluent Saos-2 human osteosarcoma cells and in differentiating normal human osteoblasts, concomitantly with the decrease of Runx2, a key transcription factor controlling bone formation. Therefore, we investigated the role of Runx2 in the regulation of BSP expression in Saos-2 cells. Using a mobility shift assay, we demonstrated that Runx2 binds to the BSP promoter only in preconfluent cells. Histone deacetylase 3 (HDAC3) has been recently shown to act as a Runx2 co-repressor. Chromatin immunoprecipitation assays demonstrated that both Runx2 and HDAC3 are detectable at the BSP promoter in preconfluent Saos-2 cells but not when they are confluent and overexpress BSP. Consistently, nuclear Runx2 protein level is down-regulated, whereas Saos-2 cells became increasingly confluent. Finally, the suppression of HDAC3, Runx2, or both by RNA interference induced the expression of BSP at both mRNA and protein levels in Saos-2 cells. Our data demonstrate that Runx2 and HDAC3 repress BSP gene expression and that this repression is suspended upon osteoblastic cell differentiation. Both the nuclear disappearance of Runx2 and the non-recruitment of HDAC3 represent new means to relieve Runx2-mediated suppression of BSP expression, thus allowing the acquisition of a fully differentiated and mineralization-competent phenotype by osteoblast cells.

Complete topographical distribution of both the in vivo and in vitro phosphorylation sites of bone sialoprotein and their biological implications

Bone sialoprotein (BSP) is a multifunctional, highly phosphorylated, and glycosylated protein with key roles in biomineralization and tissue remodeling. This work identifies the complete topographical distribution and precise location of both the in vitro and in vivo phosphorylation sites of bovine BSP by a combination of state-of-the-art techniques and approaches. In vitro phosphorylation of native and deglycosylated BSPs by casein kinase II identified seven phosphorylation sites by solid-phase N-terminal peptide sequencing that were within peptides 12-22 (LEDS(P)EENGVFK), 42-62 (FAVQSSSDSS(P)EENGNGDS(P)S(P)EE), 80-91 (EDS(P)DENEDEES(P)E), and 135-145 (EDES(P)DEEEEEE). The in vivo phosphorylation regions and sites were identified by use of a novel thiol reagent, 1-S-mono[(14)C]carboxymethyldithiothreitol. This approach identified all of the phosphopeptides defined by in vitro phosphorylation, but two additional phosphopeptides were defined at residues, 250-264 (DNGYEIYES(P)ENGDPR), and 282-289 (GYDS(P)YDGQ). Furthermore, use of native BSP and matrix-assisted laser desorption ionization time-of-flight mass spectrometry identified several of the above peptides, including an additional phosphopeptide at residues 125-130 (AGAT(P)GK) that was not defined in either of the in vitro and in vivo studies described above. Overall, 7 in vitro and 11 in vivo phosphorylation sites were identified unequivocally, with natural variation in the quantitative extent of phosphorylation at each in vivo phosphorylation site.

In vitro differentiation of embryonic stem cells into mineralized osteoblasts

Embryonic stem cells are pluripotent cells derived from the inner cell mass of mouse blastocysts that have been shown to differentiate spontaneously into cell types representing all three germ layers. This study shows that ES cells were induced to differentiate in vitro into mineralized osteoblasts under the influence of ascorbic acid, beta-glycerophosphate and 1alpha,25-OH vitamin D3. The activity of alkaline phosphatase, an early osteoblast marker, was found to be increased around day 12 of culture. Mineralized cells were clearly identified by histochemical staining, which detects mineralized calcium. The major noncollagenous component of bone matrix, osteocalcin, was localized to the mineralized cells by immunofluorescence. The expression of bone-specific genes was analyzed by real-time quantitative PCR. Osteocalcin and bone sialoprotein (BSP) were identified as early as in the fourth week of embryonic stem cell culture, both being characteristic for late stages of osteoblastic differentiation, indicating that at this time of culture the identified cells represent "mature" osteoblasts. The osteoblast-specific transcription factor Cbfa1 was induced a few days earlier. The expression of osteopontin and osteonectin, both being involved in binding calcium ions and hydroxyapatite during mineralization processes, as well as of collagen type I, representing by far the most predominant collagen in vertebrate organisms, is enhanced at the beginning of the second culture week upon addition of supplements. In the third week of culture, treated cells showed a second peak of osteopontin, osteonectin and collagen type I expression, osteopontin and osteonectin being stimulated 3-4-fold and collagen type I being induced 6-fold over control values. Alkaline phosphatase (ALP) expression was enhanced at the beginning of the third week of culture and was found to be increased again at later stages of culture at days 27-34. The in vitro differentiation of mouse embryonic stem cells into osteoblasts may provide a suitable model for studying the molecular processes of osteoblastic development in vivo.

Phenotypic effects of continuous or discontinuous treatment with dexamethasone and/or calcitriol on osteoblasts differentiated from rat bone marrow stromal cells

Osteoblasts are target cells for glucocorticoids and calcitriol, and their phenotype is greatly modified by these hormones. We investigated the effect of continuous or discontinuous hormonal exposure to osteoblasts derived from rat bone marrow stromal cells in long-term subcultures. Stromal cells were grown in primoculture in presence of dexamethasone (dex), but in following subcultures, dex and/or calcitriol were added just after seeding or after a 7-day hormone-free period. Cell proliferation, alkaline phosphatase (ALP) histochemical staining, and enzymatic bioactivity measurement, osteocalcin (OC), ALP and bone sialoprotein (BSP) mRNA expression were used to study the differential effect on osteoblastic phenotype of various conditions of treatment by dex and calcitriol. In primoculture, the osteoblastic differentiation was confirmed by the formation of calcified nodules and by strong expression of ALP, OC, and BSP mRNAs. In subcultures, proliferation of stromal cells was stimulated by dex and inhibited by calcitriol and by both hormones. Cell proliferation was not modified by hormonal lack during 7 days. Continuous hormonal treatment by dex strongly enhanced OC and BSP mRNAs, but apparently did not modified ALP mRNAs expression. Continuous treatment by calcitriol decreased ALP and the dex-induced BSP expression and stimulated the OC mRNAs level, strongly when associated with dex. The population of ALP+ cells and ALP bioactivity were strongly increased by dex, whereas calcitriol or both hormones decreased them. When the subcultures were undergone without hormonal treatment during 7 days, all osteogenic mRNAs strongly decreased even after hormonal recovery. Dex, calcitriol, and both hormones inhibited ALP mRNAs. OC messengers were only weakly detectable with both hormones. ALP+ cell population and ALP bioactivity were decreased after 14 days of hormonal treatment recovery. These results support that continuous presence of glucocorticoids appears as a major key for the permanent expression of the osteoblastic phenotype that is inhibited by calcitriol, in the rat bone marrow.

Antiproliferative action of vitamin D

During the past few years, it has become apparent that vitamin D may play an important role in malignant transformation. Epidemiological studies suggest that low vitamin D serum concentration increases especially the risk of hormone-related cancers. Experimentally, vitamin D suppresses the proliferation of normal and malignant cells and induces differentiation and apoptosis. In the present review we discuss the mechanisms whereby vitamin D regulates cell proliferation and whether it could be used in prevention and treatment of hyperproliferative disorders like cancers.

The frequency of common progenitors for adipocytes and osteoblasts and of committed and restricted adipocyte and osteoblast progenitors in fetal rat calvaria cell populations

In fetal rat calvaria (RC) cell populations, adipocyte differentiation is stimulated by both dexamethasone (Dex) and 1,25 dihydroxyvitamin D3 [1,25(OH)2D3], whereas osteoblast differentiation is stimulated by Dex but inhibited by 1,25(OH)2D3. We examined whether the osteoblastic and adipocytic colonies were derived from a common progenitor, from committed and restricted adipocyte and osteoblast progenitors, or from both and whether the adipocyte progenitors stimulated by 1,25(OH)2D3 constitute a population of progenitors that is different from that stimulated by Dex. RC cells were isolated by sequential enzyme digestion yielding five populations designated I-V. In population I the effect of Dex on adipocyte formation was greater than that of 1,25(OH)2D3, whereas the effect of 1,25(OH)2D3 was greater than that of Dex in populations III-V. We next applied replica plating techniques to further investigate the response characteristics of individual osteoprogenitors and adipocyte progenitors by looking at the fate of duplicate colonies derived from the same progenitor under different culture conditions. RC cells were plated at 1,000-1,500 cells/100 mm culture dish and a 17-microm mesh polyester membrane overlaid onto master dishes on day 4 or day 5 and removed on day 11 or day 12. Then, replicas and master dishes were cultured separately in medium containing either Dex, 1,25(OH)2D3, or Dex plus 1,25(OH)2D3 for a further 17-21 days and then fixed and stained with both Sudan IV and the von Kossa technique. Nine hundred twenty-seven matched colonies present on both master dishes and replica membranes were screened and colonies were classified as either adipocytic, osteoblastic (bone or osteoid), or fibroblastic. Results show convincingly that most of the osteoprogenitors present in fetal RC cells are committed and restricted to the osteoblastic cell lineage (95.29%); that the 1,25(OH)2D3-responsive adipocyte progenitors are different from the Dex-responsive adipocyte progenitors, but both are restricted to form adipocytes and finally; and that a common osteoblastladipocyte progenitor is present in a low frequency (4.71% of osteoprogenitors).

Structural characterization of human recombinant and bone-derived bone sialoprotein. Functional implications for cell attachment and hydroxyapatite binding

Human bone sialoprotein (BSP) comprises 15% of the total noncollagenous proteins in bone and is thought to be involved in bone mineralization and remodeling. Recent data suggest a role for BSP in breast cancer and the development of bone metastases. We have produced full-length recombinant BSP in a human cell line and purified the protein from human bone retaining the native structure with proper folding and post-translational modifications. Mass spectrometry of bone-derived BSP revealed an average mass of 49 kDa and for recombinant BSP 57 kDa. The post-translational modifications contribute 30-40%. Carbohydrate analysis revealed 10 different complex-type N-glycans on both proteins and eight different O-glycans on recombinant BSP, four of those were found on bone-derived BSP. We could identify eight threonines modified by O-glycans, leaving the C terminus of the protein free of glycans. The recombinant protein showed similar secondary structures as bone-derived BSP. BSP was visualized in electron microscopy as a globule linked to a thread-like structure. The affinity for hydroxyapatite was higher for bone-derived BSP than for recombinant BSP. Cell adhesion assays showed that the binding of BSP to cells can be reversibly diminished by denaturation.

Functional analysis of bone sialoprotein: identification of the hydroxyapatite-nucleating and cell-binding domains by recombinant peptide expression and site-directed mutagenesis

Mammalian bone sialoprotein (BSP) is a mineralized tissue-specific protein containing an RGD (arginine-glycine-aspartic acid) cell-attachment sequence and two distinct glutamic acid (glu)-rich regions, with each containing one contiguous glu sequence. These regions have been proposed to contribute to the attachment of bone cells to the extracellular matrix and to the nucleation of hydroxyapatite (HA), respectively. To further delineate the domains responsible for these activities, porcine BSP cDNA was used to construct expression vectors coding for two partial-length recombinant BSP peptides: P2S (residues 42-87), containing the first glutamic acid-rich domain; and P1L (residues 69-300), containing the second glutamic acid-rich region and the RGD sequence. These peptides were expressed in Escherichia coli as his-tag fusion proteins and purified by nickel affinity columns and FPLC chromatography. Digestion with trypsin released the his-tag fusion peptide, which generated P2S-TY (residues 42-87) and P1L-TY (residues 132-239). Using a steady-state agarose gel system, P2S-TY promoted HA nucleation, whereas P2S, P1L, and P1L-TY did not. This implies that the minimum requirement for nucleation of HA resides within the amino acid sequence of the first glutamic acid-rich domain, whereas the second glutamic acid-rich domain may require posttranslational modifications for activity. P1L, but not P2S, promoted RGD-mediated attachment of human gingival fibroblasts in a manner similar to that of native BSP. Deletion of the RGD domain or conversion of it to RGE (arginine-glycine-glutamic acid) abolished the cell-attachment activity of P1L. This suggests that, at least for human gingival fibroblasts, the major cell-attachment activity in the recombinant BSP peptides studied (residues 42-87 and 69-300) requires the RGD sequence located at the C-terminal domain.

Bone morphogenetic proteins secreted by breast cancer cells upregulate bone sialoprotein expression in preosteoblast cells

It is well established that bone metastases comprise bone; however, the exact factors/mechanisms involved remain unknown. We hypothesized that tumor cells secreted factors capable of altering normal bone metabolism. The aims of the present study were to (1) determine the effects of secretory products isolated from HT-39 cells, a human breast cancer cell line, on osteoprogenitor cell (MC3T3-E1 cells) behavior, and (2) identify tumor-derived factor(s) that alters osteoblast activities. Conditioned media (CM) from HT-39 cells were collected following a 24-h serum-free culture. The ability of CM to alter gene expression in MC3T3-E1 cells was determined by Northern analysis. CM effects on cell proliferation and mineralization ability were determined using a Coulter counter and von Kossa stain, respectively. MC3T3-E1 cells were treated with CM plus noggin, a factor known to block bone morphogenic proteins (BMPs), to determine whether BMPs, shown to be present in CM, were linked with CM effects on MC3T3-E1 cell activity. In addition, inhibitors of MAP kinase kinase (MEK), protein kinase C (PKC), and protein kinase A were used to identify the intracellular signaling pathway(s) by which the active factors in CM regulated osteoblast behavior. CM treatment significantly enhanced BSP mRNA (2.5-fold over control), but had no effect on cell proliferation. Mineralization assay showed that CM enhanced mineral nodule formation compared to controls. Noggin inhibited CM-induced upregulation of BSP mRNA, suggesting that BMPs were responsible for upregulating BSP gene expression in MC3T3-E1 cells. The PKC inhibitor blocked CM-mediated upregulation of BSP, suggesting involvement of the PKC pathway in regulating BSP expression. BMPs secreted by HT-39 cells may be responsible for enhancing BSP expression in MC3T3-E1 cells. Continued studies targeted at determining the role of BMPs in regulating bone metabolism are important for understanding the pathogenesis of bone diseases.

In vitro and in vivo evidence for orphan nuclear receptor RORalpha function in bone metabolism

Bone is a major target site for steroid hormone action. Steroid hormones like cortisol, vitamin D, and estradiol are responsible for principal events associated with bone formation and resorption. Over the past decade, new members of the nuclear hormone gene family have been identified that lack known ligands. These orphan receptors can be used to uncover signaling molecules that regulate yet unidentified physiological networks. In the present study the function of retinoic acid receptor-related orphan receptor (ROR) alpha in bone metabolism has been examined. We showed that RORalpha and RORgamma, but not RORbeta, are expressed in mesenchymal stem cells derived from bone marrow. Interestingly, for RORalpha we observed an increased messenger signal expression between control cells and cells undergoing osteogenic differentiation. Furthermore, the direct activation of mouse bone sialoprotein by RORalpha, typically 7-fold, has been shown. In contrast, transient overexpression of RORalpha overrides the activation of the osteocalcin promoter by 1alpha,25-dihydroxyvitamin D(3). In addition, we have investigated bone mass parameters and bone geometry in the mouse mutant staggerer (sg/sg), a mouse strain that carries a deletion within the RORalpha gene. Homozygote mutants have thin long bones compared with the heterozygote animals and wild-type littermates. More interestingly, the bones of the sg/sg animals are osteopenic as indicated by the comparison of bone mineral contents of sg/sg animals to the heterozygote and wild-type animals. We conclude that these in vitro and in vivo results suggest a function for RORalpha in bone biology. RORalpha most likely acts by direct modulation of a bone matrix component.

Bone sialoprotein

The search for a protein nucleator of hydroxyapatite crystal formation has been a focus for the isolation and characterization of the major non-collagenous proteins in bone. Of the proteins characterized to date, bone sialoprotein (BSP) has emerged as the only bona fide candidate for nucleation. BSP is a highly glycosylated and sulphated phosphoprotein that is found almost exclusively in mineralized connective tissues. Characteristically, polyglutamic acid and arginine-glycine-aspartate (RGD) motifs with the ability to bind hydroxyapatite and cell-surface integrins, respectively, have been conserved in the protein sequence. Expression of the BSP gene, which is induced in newly formed osteoblasts, is up-regulated by hormones and cytokines that promote bone formation and down-regulated by factors that suppress bone formation. Thus, BSP has the biophysical and chemical properties of a nucleator, and its temporo-spatial expression coincides with de novo mineralization in bone and cementum. Moreover, BSP has been associated with mineral crystal formation in several pathologies, including breast carcinomas. However, the ability of BSP to mediate cell attachment and to signal through the RGD motif points to alternate functions for BSP which need further investigation. In combination, the hydroxyapatite-binding polyglutamic acid sequences and the RGD provide bi-functional entities through which BSP may mediate the targeting and attachment of normal and metastasizing cells to the bone surface.

Altered expression of bone sialoproteins in vitamin D-deficient rBSP2.7Luc transgenic mice

Bone sialoprotein (BSP) and osteopontin (OPN) are two major noncollagenous matrix proteins in mineralized connective tissue that have discrete roles in bone matrix formation, mineralization, and remodeling. The osteotropic secosteroid, 1,25-dihydroxyvitamin D3, a potent regulator of bone remodeling required for normal bone development, has been shown to exert differential effects on OPN and BSP expression by bone cells in vitro. To investigate these effects in vivo, we induced vitamin D3 deficiency in a transgenic mouse line (rBSP2.7Luc) that has a 2.7 kb rat BSP promoter linked to a luciferase reporter gene in its genome. Pregnant rBSP2.7Luc mice were fed vitamin D3-deficient food and demineralized water for 6 weeks. Their offspring were weaned at 3 weeks of age and then fed vitamin D-deficient food for an additional week. The control group were fed normal rodent pellets and water during the entire experimental procedure. Bone tissues from 40, 4-week-old offspring in each group were analyzed for BSP, OPN and luciferase expression. Vitamin D3-deficient mice displayed a rachitic phenotype that included reduced size and malformation of bones. Assays of the BSP promoter transgene in calvariae, mandibles, and tibiae of the rachitic mice showed increases in luciferase activity of 3.1-, 1.9-, and 4.6-fold, respectively, when compared with control littermates. Semiquantitative reverse transcriptase polymerase chain reaction assays of BSP mRNA revealed increases of 7-, 74-, and 66-fold, respectively, in the same rachitic bones, while OPN mRNA was reduced 12.5-fold in calvariae and 2-fold in tibiae and mandibles. In situ hybridization using mouse cRNA probes revealed that the increased BSP expression and decreased OPN expression in the vitamin D3-deficient mice was primarily in osteoblastic cells on the surface of calvariae and endosteal spaces of alveolar bone, on newly formed epiphyseal bone, and in cementoblasts and in hypertrophic chondrocytes. These studies are the first to show that BSP and OPN are differentially regulated by vitamin D3 in vivo, reflecting the diverse roles of these protein in bone remodeling. Moreover, the increased expression of the BSP transgene in the rachitic mice demonstrates that vitamin D3 regulation of BSP expression is mediated, in part, by element(s) within the 2.7 kb promoter region.

1,25-Dihydroxyvitamin D3 upregulates natriuretic peptide receptor-C expression in mouse osteoblasts

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3], a key regulator of mineral metabolism, regulates expression of several genes related to bone formation. The present study examined the 1,25(OH)2D3-mediated regulation of natriuretic peptide receptor-C (NPR-C) expression in osteoblasts. 1,25(OH)2D3 treatment significantly increased NPR-C-dependent atrial natriuretic peptide-binding activity and synthesis of the NPR-C protein in mouse osteoblastic cells in a cell-specific manner. Western blot analysis also demonstrated that 1, 25(OH)2D3 upregulated expression of NPR-C protein in slow kinetics. Next, Northern blot analysis revealed a significant increase in the steady-state NPR-C mRNA level by 1,25(OH)2D3. Sequence analysis of the 9 kb of the 5'-flanking region of the mouse NPR-C gene revealed an absence of consensus vitamin D-response elements, and promoter analysis using osteoblastic cells stably transfected with mouse NPR-C promoter-reporter constructs showed a slight increase of promoter activity with 1,25(OH)2D3 treatment. In addition, a nuclear run-on assay exhibited that the transcriptional rate of the NPR-C gene was unchanged by 1,25(OH)2D3, whereas that of the osteopontin gene was increased. Evaluation of NPR-C mRNA half-life demonstrated that 1,25(OH)2D3 significantly increased the NPR-C mRNA stability in osteoblastic cells. 1,25(OH)2D3 attenuated intracellular cGMP production in osteoblastic cells stimulated by C-type natriuretic peptide (CNP) without a significant change of the natriuretic peptide receptor-B mRNA level, suggesting enhancement of the clearance of exogenously added CNP via NPR-C. Furthermore, NPR-C and osteopontin mRNAs in mouse calvariae were significantly increased by administration of 1,25(OH)2D3, and immunohistological analysis demonstrated that NPR-C is actually and strongly expressed in mouse periosteal fibroblasts. These findings suggest that 1,25(OH)2D3 can play a critical role for determination of the natriuretic peptide availability in bones by regulation of NPR-C expression through stabilizing its mRNA.

1,25-Dihydroxyvitamin D3 increases in vitro vascular calcification by modulating secretion of endogenous parathyroid hormone-related peptide

BACKGROUND

A significant association between vascular calcification and osteoporosis has been noted, suggesting that calcium homeostasis is important in vascular calcification as well as in osteoporosis. Moreover, results of our previous studies suggest that calcium-regulating hormones such as parathyroid hormone-related peptide (PTHrP) may modulate vascular calcification. Therefore, we hypothesized that 1alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] may have a direct impact on the calcium-regulating system of vascular smooth muscle cells, resulting in deposition of calcium in vascular wall.

METHODS AND RESULTS

We investigated the effect of 1,25(OH)2D3 on in vitro calcification by bovine vascular smooth muscle cells (BVSMCs). 1,25(OH)2D3 dose dependently increased BVSMC calcification and alkaline phosphatase activity. 1,25(OH)2D3 also decreased secretion of PTHrP by BVSMCs in a dose-dependent manner and depressed its gene expression. Furthermore, exogenous PTHrP (fragment 1-34) antagonized the stimulatory effect of 1,25(OH)2D3 on BVSMCs. Finally, 1,25(OH)2D3 dose dependently increased the expression of the osteopontin gene, one of the bone matrix proteins in BVSMCs, contributing to its stimulatory action on BVSMC calcification.

CONCLUSIONS

These data suggest that 1,25(OH)2D3 exerts a stimulatory effect on vascular calcification through direct inhibition of the expression of PTHrP in BVSMCs as an endogenous inhibitor of vascular calcification. Moreover, the stimulatory effects of 1,25(OH)2D3 on alkaline phosphatase activity and osteopontin expression may contribute to its promoting action in vascular calcification.

Evidence for phosphoprotein microspheres in bone

Bone sialoprotein and osteopontin are 'bone-specific' phosphoproteins, but their function is uncertain and their ultrastructural associations remain unclear. Insight into their role was sought by special attention to their general distribution and specific morphology under the high-power optical microscope. Their extracellular staining characteristics were examined in cryosections of adult rat skeletal tissues using two immunohistochemical methods. The two proteins were clearly evident in immature woven bone of endochondral and intramembranous origin (although cartilage was negative, even when calcified). In mature lamellar bone, bone sialoprotein remained ubiquitous, while osteopontin was confined to cement lines and other relatively discrete sites of past and present resorption activity, particularly near blood vessels. In neither case was the distribution of the stain structureless and diffuse. Invariably (except when non-specific), it was sharply defined and had the form of microspheres measuring approximately 1 microm in diameter. In both immature and mature regions, these objects appeared in sheets, chains or groups in a pattern that was evidently coincident with a similar structural arrangement found within the inorganic phase of bone. It was concluded that phosphoproteins are not randomly located throughout the collagenous matrix but are apparently integral to calcified microsphere populations, and it is suggested that these structures are well placed to control the chemical state of the mineral over their surfaces and influence remodelling.

Serum concentrations of cartilage oligomeric matrix protein and bone sialoprotein in hip osteoarthritis: a one year prospective study

OBJECTIVE

To evaluate serum concentrations of cartilage oligomeic matrix protein (COMP) and bone sialoprotein (BSP) as predictors of disease progression in hip osteoarthrtitis (OA).

METHODS

Forty eight consecutive patients, referred to hospital for symptomatic hip OA, (ACR criteria) were monitored in a one year prospective trial with radiographs and serum samples. The radiographs were graded for joint space narrowing, osteophytes, and sclerosis and the joint space width was measured by a digitised image analyser. Serum COMP and BSP were quantified by immunoassays.

RESULTS

The COMP concentrations at baseline correlated with the joint space width at entry and with its yearly mean narrowing (r = 0.38, p = 0.002) but not with joint space narrowing grade progression. The concentrations were higher in patients with bilateral hip OA (p = 0.03). The serum BSP concentrations at baseline were unrelated to OA progression but correlated inversely to the osteophyte grade (r = -0.36, p = 0.004) and sclerosis grade (r = -0.42, p = 0.0004).

CONCLUSION

Serum COMP seems to be a surrogate marker of OA and may be of interest for the detection of patients at risk of rapidly progressing disease in hip OA. Serum BSP changes seem to reflect alterations in the subchondral bone turnover in hip OA. Measurement of joint space width using a digitised image analyser is a sensitive way of assessing OA progression that facilitates evaluation of tissue markers in relation to anatomical changes in the joint.

Tissue specific and vitamin D responsive gene expression in bone

Studies of gene expression in bone have adopted a number of molecular approaches that seek to determine those cis and trans-acting factors responsible for the development and physiological regulation of this unique tissue. The majority of studies have been performed in vitro, focussing on the expression of genes such as osteocalcin, bone sialoprotein and type I collagen which demonstrate restricted or altered expression patterns in osteoblasts. These studies have demonstrated a large number of cis and trans acting factors that modulate the tissue specific and vitamin D responsive expression of these genes. These include the response elements and regions mediating basal and vitamin D dependent transcription of these genes as well as some of the transcription factors that bind to these regions and the nucleosomal organisation of these genes within a nuclear framework. In vivo studies, including the introduction of transgenes into transgenic mice, extend these in vitro observations within a physiological context. However, in part due to limitations in each approach, these in vitro and in vivo studies are yet to accurately define all the necessary cis and trans-acting factors required for tissue specific and vitamin D responsive gene expression. Advances have been made in identifying many cis-acting regions within the flanking regions of these genes that are responsible for their restricted expression patterns, but a vector incorporating all the necessary cis-acting regions capable of directing gene expression independent of integration site has not yet been described. Similarly, trans-acting factors that determine the developmental destiny of osteoblast progenitors and the restricted expression of these genes remain elusive and, despite advances in the understanding of protein-DNA interactions at vitamin D response elements contained within these genes, further intermediary factors that interact with the transcriptional machinery to modulate vitamin D responsiveness need to be identified.

Expression of osteonectin mRNA in human breast tumours is inversely correlated with oestrogen receptor content

Osteonectin is a secreted glycoprotein which is detected in a number of normal and neoplastic human tissues in vivo. It is an extracellular matrix (ECM)-associated protein which is postulated to regulate cell migration, adhesion, proliferation and matrix mineralisation and previous reports suggest that it may be modulated by steroid hormones in target tissues. The aim of this study was to measure osteonectin mRNA and protein expression in breast tumour biopsies and compare these with oestrogen (ER) and progesterone receptor (PR) levels in the same tumours. An inverse correlation was seen between osteonectin mRNA expression and ER level. Samples with low ER protein expression had a mean osteonectin mRNA level which was almost 4-fold greater than the mean level of expression observed in tumours containing high concentrations of ER protein. This inverse correlation was statistically significant. Despite the strong inverse relationship between osteonectin mRNA levels and tumour ER content, no correlation was seen when osteonectin protein concentration was measured in tumour cytosols on immunoblots and compared to ER and PR levels in the same tumours. However, since it is a secreted protein, osteonectin protein expression may not reflect cellular osteonectin levels in breast tumours. In summary, these data suggest that ER-mediated suppression of osteonectin gene expression may contribute to the less aggressive characteristics associated with receptor-positive tumours and that loss of ER expression may lead to over-expression of osteonectin and contribute to a poorer differentiated, more invasive phenotype.

Glucocorticoid-dependent transcriptional repression of the osteocalcin gene by competitive binding at the TATA box

The human osteocalcin gene is transcriptionally repressed by glucocorticoids. A specific binding element for the glucocorticoid receptor (GR) overlapping the TATA box of the human osteocalcin promoter has previously been identified. In the present study, the function of this element has been further characterized by competitive gel mobility-shift assay and transfection experiments. The GR and TATA-binding protein (TBP) bound to the cognate overlapping elements in a mutually exclusive manner. The GR preferentially inhibited the binding of TBP. The isolated DNA-binding domain of the GR is sufficient to compete for TBP binding. The integrity of both half-sites of the glucocorticoid response element (GRE) is required to effectively compete for TBP binding, and competitive binding of the GR is dependent on dimerization. Transient overexpression of TBP overrides the transcriptional repression of the osteocalcin promoter by glucocorticoids. We conclude that the repressive effect of glucocorticoids on this promoter is the result of competitive DNA binding to a basal transcriptional element and that it does not appear to require direct protein-protein interaction between the competitive factors.

Attachment of osteoblastic cells to hydroxyapatite crystals by a synthetic peptide (Glu7-Pro-Arg-Gly-Asp-Thr) containing two functional sequences of bone sialoprotein

We investigated activity of bone sialoprotein (BSP) to mediate attachment of cells to hydroxyapatite using a model peptide, Glu7-Pro-Arg-Gly-Asp-Thr, which contains a putative hydroxyapatite-binding site (poly-Glu) and a cell-attachment site. The peptide has affinity to hydroxyapatite with a dissociation constant of 13.5 microM. The peptide affected in vitro mineralization in a gel system, indicating interaction between this peptide and calcium phosphate. The osteoblastic cell line MC3T3-E1 was incubated with hydroxyapatite powder coated with the peptide or proteins. Attachment of the cells was observed on the powder coated with BSP, but not on the powder coated with serum albumin. The cells were attached to the powder coated with the peptide. The cells were flattened on the powder, and pseudopods developed. The attachment of the cells was inhibited by an excessive amount of Gly-Arg-Gly-Asp-Ser peptide. In conclusion, BSP mediated attachment of osteoblastic cells to hydroxyapatite, and this activity could be accomplished only by the poly-Glu sequence and the Arg-Gly-Asp sequence.

1,25-dihydroxyvitamin D3 inhibits Osteocalcin expression in mouse through an indirect mechanism

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3), a key regulator of mineral metabolism, regulates the expression of several genes that are expressed in osteoblasts. In particular, in rat and human osteoblasts, 1,25-(OH)2D3 increases the expression of Osteocalcin by interacting, through a hormone-receptor complex, with a vitamin D-responsive element present in the promoter of the genes. Here we show that in mouse, 1,25-(OH)2D3 inhibits the expression of both osteocalcin genes, OG1 and OG2. This inhibition was observed in primary osteoblast cultures and in the whole animal. From sequence inspection, DNA transfection experiments, and DNA binding assays, we could not identify a functional vitamin D-responsive element in the promoter of OG2 or in the first 3.3 kilobases of the OG1 promoter. However, we show that 1,25-(OH)2D3 treatment of primary osteoblasts abolishes the binding of OSF2, an osteoblast-specific activator of transcription that binds to OSE2, a critical osteoblast-specific cis-acting element present in OG1 and OG2 promoters. Consistent with these DNA binding data, a mutation in OSE2 in the OG2 promoter abrogated the inhibitory effect of 1,25-(OH)2D3 treatment on this promoter activity. This study illustrates that 1,25-(OH)2D3 can play different roles in the expression of the same gene in various species and indicates that this regulation in mouse occurs through an indirect mechanism, 1,25-(OH)2D3 acting on a gene genetically located upstream of Osteocalcin.

Structural analysis and characterization of tissue and hormonal responsive expression of the avian bone sialoprotein (BSP) gene

Bone sialoprotein (BSP) is an extracellular matrix protein that has a highly restricted expression to mineralized skeletal tissues. The chicken bone sialoprotein-encoding gene (bsp) was isolated and shown to contain two less exons than similar mammalian genes, with the absence of an untranslated 5' exon and the fusion of the first two exons that encode the signal peptide and amino terminal end of the mature BSP peptide. Primer extension analysis showed one strong transcriptional start point (tsp) in mRNA prepared from embryonic bone. Comparison of the avian bsp promoter sequence to those of other genes expressed in vertebrate skeletal tissues, identified the presence of homeobox protein binding sequence motifs for engrailed (en-1) and Msx 2 (Hox 8.1), and two collagen type II gene silencer elements. Two TATA sequences one at -21 bp and the second at -172 bp to the tsp were identified. For the first TATA element no CCAAT sequence was observed at an appropriate cis position however two Sp1 sequences (GGGCGG) were identified at -66 and -85 bp. A CCAAT element was seen in an appropriate cis position in relationship to the second upstream TATA, but transient expression analysis in embryonic chicken calvaria osteoblasts using two separate promoter/reporter constructs (+24 to -1244 bp or -121 to -1244 bp), confirmed that only the proximal TATA and Sp1 elements were functional. The +24 to -1244 bp promoter sequence demonstrated 33.6, 13.2, and 3.2 fold activity above base line respectively, within cells prepared from embryonic chicken calvaria bone, cephalic sterna, a cartilage that undergoes mineralization and caudal sterna, a cartilage that does not mineralize during embryogenesis. Only base line activity was observed within cells prepared from embryonic dermal fibroblasts a non-skeletal tissue, which does not express BSP. These same cells demonstrated comparable steady state mRNA levels, corroborating that this segment of promoter DNA had tissue specific activity. A series of nested deletions from the 5' end of the -1244 construct demonstrated that a portion of the tissue specific regulation was controlled by the presence of a silencer element(s) between -1244 and -620 bp since deletion of this segment of DNA resulted in a 6 fold increase in the promoter activity in dermal skin fibroblasts. The -1244-(+)24 nt promoter construct was shown to be stimulated by dexamethasome approximately 1.5 fold over control, inhibited by 1,25(OH)2D3 approximately 60% of control and was strongly stimulated approximately 5.0 fold by parathyroid hormone (PTH) in embryonic calvaria osteoblasts. These data define the proximal promoter of the avian bsp gene and identify several potential regulatory elements that have been observed in the promoters of other genes expressed in skeletal tissues. These elements imparted both tissue and hormone specific promoter activity to bsp expression within skeletal cells.

Expression and regulation of galectin 3 in rat osteoblastic cells

Galectin 3 is an endogenous soluble beta-galactoside-specific lectin originally identified and termed epsilon BP or IgE-binding protein in rat basophilic leukemia cells, but its wide tissue distribution and the multiple contexts in which it has been isolated have suggested that its function may not be limited to IgE binding but may include a role in cell growth regulation and differentiation, neoplastic transformation, and cell adhesion (Liu, 1990, Crit. Rev. Immunol., 10:289-306; Barondes et al., 1994, J. Biol. Chem., 269:20807-20810). After immunoscreening of a lambda gt11 cDNA expression library made from bone-nodule forming cultures of fetal rat calvaria (RC) cells with an antibody raised against osteoblastic cells (Turksen et al., 1992, J. Histochem. Cytochem., 40:1339-1352), three cDNA clones were isolated and sequenced; the sequence matched that of rat galectin 3. Galectin 3 mRNA was detected in various fetal and adult rat tissues, including calvaria and cultured RC cells. In RC cells and the rat osteosarcoma cell line ROS 17/2.8, galectin 3 mRNA expression increased with time in culture, in contrast to its behavior in fetal rat skin fibroblasts (RSF) in which its expression decreased with time in culture. In a second rat osteosarcoma line, UMR 106.01, galectin 3 mRNA was almost nondetectable. The synthetic glucocorticoid dexamethasone (Dex) enhanced galectin 3 expression in RSF cell cultures, while 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) had no significant effect. In contrast, Dex downregulated and 1,25(OH)2D3 upregulated galectin 3 expression in RC and ROS 17/2.8 cells, especially at later time points in culture when expression of osteoblast-associated differentiation markers by these cell types is most marked. Immunolabeling with an antibody against rat galectin 3 to identify galectin 3 protein showed that cells labelled within both the ROS 17/2.8 and RC populations but with marked intercellular heterogeneity of intensity. Our data support the conclusion that galectin 3 is a previously unrecognized product of osteoblastic cells, that galectin 3 mRNA and protein expression increases with time in vitro concomitant with other markers of osteogenesis, including formation of bone nodules and expression of osteoblast-associated markers such as alkaline phosphatase, bone sialo-protein, and osteocalcin, and that its expression is regulated by hormones such as glucocorticoids and 1,25(OH)2D3 that modulate other aspects of the osteoblast phenotype.

Dexamethasone regulation of marrow stromal-derived osteoblastic cells

The clonal subtypes of cells in the osteogenic family represented by fibroblastoid MBA-15.33, preosteoblast MBA-15.4, and mature osteoblastic MBA-15.6 cells were used to study the effects of glucocorticoid (dexamethasone). The role of dexamethasone was monitored on cell attachment when plated on various protein substrata (BSA, collagen 1, and Matrigel). A 24 h exposure of the cells to 10(-6) M or 10(-7) M dexamethasone differential affects their attachment preference. MBA-15.33 and MBA-15.4 cells increased their attachment capability on collagen 1, while MBA-15.6 cells' attachment was inhibited. Pretreatment with (10(-6) M) dexamethasone caused an increase in attachment on Matrigel by MBA-15.33 cells and to less extent by MBA-15.4 cells. Additionally, measurements of two enzymatic activities were monitored; one is alkaline phosphatase (ALK-P), and the second is neutral endopeptidase (CD10/NEP). MBA-15.33, MBA-15.4, and MBA-15.6 cells were exposed to dexamethasone or to various growth factors (bone morphogenic protein (BMP-2 and BMP-3), TGF beta, and IGF-1). In some experiments, pretreatment of cells by dexamethasone was followed by exposure to the growth factors. The cells' challenged cellular responses were not uniform and revealed a differential pattern when their ALK-P and CD10/NEP enzymatic activities were measured.

Bone sialoprotein and osteopontin distribution at the osteocartilaginous interface

In situ hybridization and ultrastructural immunohistochemistry were used to study the synthesis and distribution of 2 bone matrix proteins, bone sialoprotein and osteopontin, in the proximal tibial epiphysis in 21-, 32-, and 84-day-old rats. Bone sialoprotein messenger ribonucleic acid expression was restricted to cells close to the osteocartilaginous interfaces, whereas osteopontin messenger ribonucleic acid was shown also in the central epiphysis. Moreover, bone sialoprotein synthesis decreased with age, while osteopontin synthesis remained unchanged. Bone sialoprotein and osteopontin immunolabeling were concentrated to a 1-micron thick zone at the border between mineralized articular cartilage and subchondral bone. Compared with the youngest animals, total bone sialoprotein immunolabeling increased in the 32-day-old group, but did not increase further in the oldest group. In contrast, osteopontin immunolabeling increased only in the oldest rats with a decelerating rate of growth. The accumulation and localization of osteopontin at osteocartilaginous interfaces differ from previous observations in metaphyseal bone where osteopontin has been found at the mineralization front of the osteoid, possibly reflecting different roles in the mineralization process in the 2 regions. The different pattern of synthesis of the 2 proteins in the epiphysis corroborates previous indications of different biological roles.

Identification of a vitamin D3-response element that overlaps a unique inverted TATA box in the rat bone sialoprotein gene

Bone sialoprotein (BSP), an early marker of osteoblast differentiation, has been implicated in the nucleation of hydroxyapatite during bone formation de novo. Our studies, using the osteoblastic cell line ROS 17/2.8, have revealed that rat BSP gene expression is suppressed by 1,25-dihydroxyvitamin D3[1,25(OH)2D3], which is a powerful regulator of bone formation and resorption. To determine the molecular basis of the transcriptional suppression of BSP gene transcription by 1,25(OH)2D3, we have conducted transient transfection analyses with chimaeric constructs of the rat BSP gene promoter linked to a luciferase reporter gene. 1,25(OH)2D3 suppressed expression in all constructs, including a short construct (pLUC 3; nt -116 to +60) that contained a putative vitamin D3-response element (VDRE; AGGGTTTATAGGTCA; nt -28 to -14) that overlaps a unique inverted TATA (TTTATA) box. Mobility shift assays demonstrated strong binding of recombinant human vitamin D3 receptor protein (hVDR) to the VDRE. Point mutations introduced into each half-site and analysed for 1,25(OH)2D3-mediated suppression of transcription and for hVDR binding either decreased or increased both transcriptional suppression and binding. In comparison with activating VDREs, the rat BSP VDRE bound VDR-VDR homodimers more avidly than VDR-RXR alpha heterodimers (where RXR is retinoid X receptor). These studies have therefore identified a novel 1,25(OH)2D3 suppressor element that overlaps the inverted TATA box in the rat BSP gene and indicate that transcriptional suppression of the rat BSP gene by 1,25(OH)2D3 might involve competition between the VDR and the TATA binding protein (TBP).

Expression of bone sialoprotein mRNA by cells lining the mouse tooth root during cementogenesis

Adhesion molecules are considered to have an active role in controlling cell differentiation, although the mechanisms involved have yet to be determined. The developing tooth provides an excellent model to use for determining the factors/processes regulating cell differentiation. The studies presented here focused specifically on the timed and spatial expression of a bone-associated adhesion molecule, bone sialoprotein, during tooth root development. Mandibular tissues in the first molar region of CD-1 mice, at sequential stages of development, were analysed by in situ hybridization. The results demonstrate distinct expression of bone sialoprotein in surrounding bone at early stages of tooth development. At stages of active cementogenesis, bone sialoprotein transcripts were specific to cells lining the root surface, with limited expression in the surrounding connective tissue (periodontal ligament) region. The strong expression of bone sialoprotein, a mineral-specific protein having the capacity to act as a nucleator of hydroxyapatite in vitro, by cells lining the root surface at early stages of cementogenesis suggests that this molecule is operative in the cell/matrix events that accompany cementum formation.

AP-1 regulation of the rat bone sialoprotein gene transcription is mediated through a TPA response element within a glucocorticoid response unit in the gene promoter

Bone sialoprotein (BSP), a protein which has been implicated in the initial mineralization of newly-formed bone, provides an early phenotypic marker for differentiated osteoblasts. BSP expression is induced by glucocorticoids in association with osteoblast differentiation, and a glucocorticoid response element (GRE) overlapping a putative TRE (TPA, 12-O-tetradecanoyl-phorbol 13-acetate, response element) site has been identified in the rat BSP promoter (Ogata et al., 1995). Since AP-1 and the glucocorticoid receptor have a central role in regulating cell proliferation and differentiation, we have studied AP-1 activity, stimulated by 100 ng/ml TPA in normal fetal rat calvarial cells and in transformed rat osteosarcoma cells (ROS 17/2.8). A transient induction of both c-fos and c-jun mRNAs by TPA was observed in both cell populations, together with an associated suppression of BSP mRNA in the fetal rat calvarial cells. Rat BSP promoter constructs, transiently transfected into ROS 17/2.8 cells, were used to show that TPA suppressed transcription of a luciferase construct (-938/+60; pLUC6) that included the GRE/TRE, but not transcription of shorter contructs lacking this element. Notably, suppression of pLUC6 transcription by TPA was abrogated in the presence of the synthetic glucocorticoid, dexamethasone. Gel mobility shift analyses were performed using two double-stranded synthetic oligonucleotides. These encompassed the TRE and either the distal pair of GRE half-sites (-936/ -910; GRE3) or the proximal pair of GRE half-sites (-925/-899; GRE 4) that comprise the GRE/AP-1 element. The assay showed binding of both AP-1 complexes and recombinant c-Jun homodimers. Additionally, either the c-Jun or glucocorticoid receptor could displace its counterpart from the GRE/TRE but not from consensus GRE and TRE oligonucleotides, indicating that the abrogation of AP-1-mediated gene suppression by glucocorticoids could involve competitive binding. These studies, therefore, have identified a glucocorticoid response unit through which c-Fos and c-Jun can suppress the expression of BSP in proliferating pre-osteoblastic cells and through which glucocorticoids can ameliorate the effects of AP-1 and promote osteoblast differentiation and the associated expression of BSP.

Expression of bone matrix proteins during dexamethasone-induced mineralization of human bone marrow stromal cells

Glucocorticoids have been shown to induce the differentiation of bone marrow stromal osteoprogenitor cells into osteoblasts and the mineralization of the matrix. Since the expression of bone matrix proteins is closely related to the differentiation status of osteoblasts and because matrix proteins may play important roles in the mineralization process, we investigated the effects of dexamethasone (Dex) on the expression of bone matrix proteins in cultured normal human bone marrow stromal cells (HBMSC). Treatment of HBMSC with Dex for 23 days resulted in a significant increase in alkaline phosphatase activity with maximum values attained on day 20 at which time the cell matrix was mineralized. Northern blot analysis revealed an increase in the steady-state mRNA level of alkaline phosphatase over 4 weeks of Dex exposure period. The observed increase in the alkaline phosphatase mRNA was effective at a Dex concentration as low as 10(-10) M with maximum values achieved at 10(-8)M. In contrast, Dex decreased the steady-state mRNA levels of both bone sialoprotein (BSP) and osteopontin (OPN) over a 4 week observation period when compared to the corresponding control values. The relative BSP and OPN mRNA levels among the Dex treated cultures, however, showed a steady increase after more than 1 week exposure. The expression of osteocalcin mRNA which was decreased after 1 day Dex exposure was undetectable 4 days later. Neither control nor Dex-treated HBMSC secreted osteocalcin into the conditioned media in the absence of 1 ,25(OH)(2)D(3) during a 25-day observation period. The accumulated data indicate that Dex has profound and varied effects on the expression of matrix proteins produced by human bone marrow stromal cells. With the induced increment in alkaline phosphatase correlating with the mineralization effects of Dex, the observed concomitant decrease in osteopontin and bone sialoprotein mRNA levels and the associated decline of osteocalcin are consistent with the hypothesis that the regulation of the expression of these highly negatively charged proteins is essential in order to maximize the Dex-induced mineralization process conditioned by normal human bone marrow stromal osteoprogenitor cells.

Regulation of bone sialoprotein and osteopontin mRNA expression by dexamethasone and 1,25-dihydroxyvitamin D3 in rat bone organ cultures

Bone sialoprotein (BSP) and osteopontin (OPN) are prominent components of the extracellular matrix of mineralized connective tissues that have been implicated in the formation and remodelling of bone. Although these proteins have similar biochemical properties and are expressed by bone cells during bone formation it has been suggested that they have different functions and that their expression is regulated independently by hormones and cytokines. The precise role of these proteins has, however, yet to be firmly established. Since steroid hormones strongly influence the formation of bone we have analyzed the effects of glucocorticoids and 1,25 dihydroxyvitamin D3 (1,25-(OH)2D3) on the expression of BSP and OPN mRNAs in developing rat bone in vitro using in situ hybridization. In these studies it has been possible to identify the nature and spatial distribution of the cells that respond to these hormones by changes in sialoprotein expression. When cultured in the presence of the synthetic glucocorticoid, dexamethasone (dex), expression of BSP mRNA by hypertrophic cartilage cells in the tibiae and mandible was dramatically increased as were the number of responding cells indicating that glucocorticoids promote differentiation of hypertrophic cartilage cells as well as osteoblasts. Dexamethasone also stimulated a marked (> 5-fold) increase in OPN expression by osteoblasts and cells lining endosteal and periosteal bone surfaces. In contrast to dex, 1,25-(OH)2D3 suppressed BSP expression in osteoblastic cells whereas OPN expression was strongly (> 5-fold) stimulated in all three cultured bone tissues. Histological examination of the tissues showed that cell viability was retained over the culture period. However, in the presence of 1,25-(OH)2D3 considerable resorption of the tissue was evident, with cement and reversal lines being prominent. The increased expression of BSP and OPN by dex is consistent with the stimulation of bone formation by glucocorticoids, whereas the differential effects of 1,25-(OH)2D3 on BSP and OPN may reflect a stimulation of bone remodelling.

Dexamethasone stimulates luciferase gene expression through the rat bone sialoprotein gene promoter in transgenic mice

Bone sialoprotein (BSP) is expressed by differentiated osteoblasts during the initial formation and mineralization of bone matrix. Studies using transgenic mice harboring 2.7 kb of the rat BSP promoter linked to a luciferase reporter gene have shown luciferase activity in bone and other mineralized tissues while most soft tissues tested expressed a much lower level of the reporter gene. To study regulation of the transgene, mice were administered dexamethasone (dex) by intramuscular injection. After 4 h and 24 h, various tissues were dissected from the treated mice as well as from untreated transgenic littermates. Luciferase assays showed that dex stimulated expression of the transgene significantly. In bone tissues, dex increased the average luciferase activity 1.6- to 11-fold compared with control tissues from untreated transgenic mice. The luciferase activity in lung, liver and kidney remained at a low level and showed no increase with dex treatment. In some animals, however, the luciferase activity in brain and skin was also increased after dex administration. These experiments indicate that a transgene comprising 2.7 kb of the rat BSP promoter linked to a luciferase reporter is regulated in a tissue and developmental stage-dependent manner and that glucocorticoid-induced stimulation of BSP gene expression may be mediated within this region of the promoter.

Expression of bone sialoprotein mRNA during bone formation and resorption induced by colchicine in rat tibial bone marrow cavity

In the rat tibial bone marrow cavity, following colchicine injection, there is a phase of osteogenesis in which bone trabeculae replace the necrotic bone marrow tissues and fill the marrow cavity. The newly formed bone is subsequently resorbed by osteoclasts and normal bone marrow is restored. In this study, we correlated these morphologic events with the pattern of gene expression of bone sialoprotein (BSP), an extracellular matrix protein in mineralized tissues, to elucidate the possible functions of BSP in bone formation and resorption in vivo. The expressions of osteopontin (OPN) and type I collagen were also examined. Northern hybridization of the tibia demonstrated that OPN mRNA was gradually increased and expressed at a maximal level 10 days after colchicine injection (during the bone resorption process), while BSP mRNA expression already reached a maximal level at day 6 (during the initial process of bone formation). Its expression was, thus, quite temporary at the beginning of bone formation and different from that of type I collagen, which was continually elevated from days 6 to 10. In situ hybridization of the newly formed bone induced in the tibia revealed that BSP mRNA was evenly expressed in most osteoblasts and osteocytes, moreover in interconnecting colonies of spindle-shaped cells, possibly preosteoblasts, at day 6. At day 10, however, its expression became restricted to some cells on the bone surfaces, some osteoblasts, and most osteoclasts. These observations suggest that BSP may play an important role mainly in the initiation of bone formation and is also associated with the functions of osteoclast in vivo.

Glucocorticoid regulation of bone sialoprotein (BSP) gene expression. Identification of a glucocorticoid response element in the bone sialoprotein gene promoter

Glucocorticoids modulate the development and growth of many organs through interactions with a specific intracellular receptor (glucocorticoid receptor) that regulates gene transcription through a cognate element, the glucocorticoid response element (GRE), in the promoter of target genes. In bone formation glucocorticoids stimulate osteoblast differentiation and the formation of bone matrix. Recent studies have demonstrated that the induction of the bone sialoprotein (BSP) gene is associated with osteoblast differentiation and de novo bone formation. To determine the molecular pathways of glucocorticoid regulation of BSP expression, we have analyzed the effects of the synthetic glucocorticoid, dexamethasone, on the expression of the BSP by bone cells in vitro. At 10 nM, dexamethasone induced BSP expression in association with bone tissue formation by confluent fetal rat calvarial cells and adult rat marrow cells and also stimulated BSP expression up to sixfold in osteoblastic cells (UMR 106-6 and ROS 17/2.8 cells). Most of the stimulation was blocked by cycloheximide, indicating direct and indirect mechanisms of BSP gene regulation. Nuclear 'run-on' transcription analysis revealed an up to twofold increase in transcription corresponding to the increase in mRNA that was unaffected by cycloheximide. Analysis of BSP mRNA in the presence of a transcription inhibitor (5,6-dichloro-1-beta-D-ribofuanosyl benzimidazole) by Northern hybridization revealed that the stability of the BSP mRNA was not significantly altered by dexamethasone, indicating that the major, indirect, stimulation of BSP expression involves a nuclear post-transcriptional mechanism. To study the direct effects of dexamethasone, nucleotide sequence analysis of the rat BSP promoter was extended upstream to position -2992 and downstream to +2282 in the first intron. Transient transfection analyses, using various rat BSP promoter constructs linked to a luciferase reporter gene, and gel mobility shift assays were used to identify a putative glucocorticoid response unit comprising three GRE half-sites and a putative AP-1 site, located within positions -906 to -931 upstream from the translation start site of the BSP gene promoter. BSP transcription was stimulated approximately 1.5-fold by dexamethasone through this GRE, indicating that its direct effects are mediated by glucocorticoid receptor binding to this site. These studies, therefore, have identified both indirect and direct pathways of glucocorticoid regulation of BSP gene expression, the direct effects being mediated by a GRE in the rat BSP promoter through which the effects of glucocorticoids on BSP gene transcription appear to be regulated.

Stimulation by bone sialoprotein of calcification in osteoblast-like MC3T3-E1 cells

Bone sialoprotein (BSP) containing an Arg-Gly-Asp cell-binding sequence was purified from bovine bone 4 M guanidine-HCl extract after HCl demineralization by a series of chromatographic procedures. When this protein was coated on culture dishes in the presence of type I collagen, it increased both DNA content and alkaline phosphatase (ALP) activity in osteoblast-like MC3T3-E1 cells, and stimulated calcification in the cells, whereas fibronectin, another cell-binding protein, showed a marked increase in the DNA content but had little effect on the ALP activity. These findings suggest that BSP is mitogenic for preosteoblasts and differentiating the cells into osteoblasts, thereby stimulating bone calcification.

Inhibitory effects of 1,25(OH)2 vitamin D3 on collagen type I, osteopontin, and osteocalcin gene expression in chicken osteoblasts

Seventeen day chicken embryonic osteoblasts treated over a 30-day period with 1,25(OH)2 D3 showed a 2-10-fold decrease in collagen, osteopontin and osteocalcin protein accumulation, alkaline phosphatase enzyme activity, and mineral deposition. Comparable inhibition in the steady state mRNA levels for alpha 1(I) and alpha 2(I) collagen, osteocalcin, and osteopontin were observed, and the inhibitory action of the hormone was shown to be specific for only the late release populations of cells from sequential enzyme digestions of the chick calvaria. In order to determine whether the continuous hormone treatment blocked osteoblast differentiation, the cells were acutely treated for 24 h with 1,25(OH)2 D3 at culture periods when the cells proliferate (day 5), a culture period when the cells cease further cell division and are increasing in the expression of their differentiated functions (day 17), and a culture period when the cells are encapsulated within a mineralized extracellular matrix (day 30). Inhibition of the expression of collagen, osteocalcin, and osteopontin were observed at days 17 and 30, while no effect could be detected for the 5-day cultures. To further define whether the inhibitory effect was specific for cells expressing their differentiated phenotype, 1,25(OH)2 D3 treatment was initiated at day 17 and continued to day 30 after the cells have established their collagenous matrix. In these experiments further collagenous matrix deposition, mineral deposition, alkaline phosphatase activity, and osteocalcin synthesis were also inhibited after the hormone treatment was initiated. These results, in summary, show that 1,25(OH)2 D3 in primary avian osteoblast cultures derived from 17-day embryonic calvaria inhibits the expression of several genes associated with differentiated osteoblast function and inhibit extracellular matrix mineral deposition.

The effect of glucocorticoids on osteoblast function. The effect of corticosterone on osteoblast expression of beta 1 integrins

Prolonged treatment with glucocorticoids is known to produce osteoporosis, which is characterized by a decrease in bone mass. Therefore, we studied the effect of glucocorticoids on the formation of bone and on the expression of beta 1 integrins in a mineralizing organ culture of fetal rat parietal bone. Integrins are a family of integral membrane glycoproteins that mediate the adhesion of cells to extracellular matrix macromolecules and affect the growth and differentiation of cells. In situ hybridization with a 32P-labeled beta 1 integrin cDNA probe was performed on parietal bone, treated with or without 100-nanomolar corticosterone for ninety-six hours, to localize and assess the levels of beta 1 integrin mRNA quantitatively. Corticosterone decreased beta 1 integrin mRNA in the osteoblast layer but not in the periosteum. Northern blot analysis demonstrated a 62 per cent decrease in the levels of beta 1 integrin mRNA in the osteoblast layer of bone that had been stripped of its periosteum. Immunofluorescence microscopy confirmed these results, as they demonstrated a decrease in the levels of beta 1 integrin protein predominantly in the osteoblast layer. This effect was dependent on the concentration of corticosterone. During ninety-six hours of culture, the calcium content and the dry weight of control parietal bone increased 157 per cent and 57 per cent, respectively. However, treatment of these cultures with 100-nanomolar corticosterone inhibited calcification by 24 per cent. The administration of glucocorticoid had no significant effect on the DNA content or dry weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Further characterization of interaction between bone sialoprotein (BSP) and collagen

Bone sialoprotein (BSP) has an affinity to collagen fibrils [25]. A role of carbohydrate chains in the affinity was examined by removing sialic acids of BSP. Neuraminidase treatment of the BSP increased the binding to collagen. Binding sites of BSP on collagen were examined by biochemical and electron-microscopic methods. Purified bovine BSP was labeled with biotin. Collagen alpha chains or CNBr peptides were separated by electrophoresis and transfered to nitrocellulose membranes. The membranes were incubated with the biotin-labeled BSP, and the bound BSP was visualized with avidin conjugated with alkaline phosphatase. The labeled BSP was preferentially bound to the alpha 2 chain, and peptides derived from alpha 2 chain. In another experiment, the labeled BSP was incubated with reconstituted native collagen fibrils. The mixture was put on a copper grid, reacted with avidin conjugated with gold particles, and observed with an electron microscope. The gold particles were seen mainly within hole zones of the fibrils. BSP bound to the alpha 2 chain within the hole zones may regulate the onset of calcification at hole zones and the cell binding to collagen fibrils.