Interaction of osteopontin with osteoclast integrins

Use of some bone-related cytokines as predictors for rheumatoid arthritis severity by neural network analysis

Background. Rheumatoid arthritis (RA) is characterized by synovial membrane inflammation that results in joint damage. Many earlier studies have measured cytokines for a better diagnosis of RA. In the present study, three bone biomarkers (osteopontin, Stromelysin-1 (MMP3), and vascular endothelial growth factor-A (VEGF)) are examined for their ability to estimate the severity of disease by using artificial neural network (NN) analysis and regression analysis. Methods: The study enrolled 87 RA patients and 44 healthy control subjects. The biomarkers were measured by the enzyme-linked immunosorbent assay (ELISA) technique. Disease Activity Score (28 joints) and C-reactive protein (CRP) (DAS28-CRP) was calculated by using (DAS28-CRP) calculator. The patients with DAS28-CRP5.1 are considered as having high disease activity (HDA). While patients group with DAS28-CRP5.1 are considered as moderate disease activity (MDA). The neural network (NN) analysis was used for the differentiation between groups. Results. Results showed that the most sensitive predictor for high disease activity (HDA) of RA is MMP3, followed by osteopontin and VEGF. These three biomarkers can differentiate significantly between HDA and moderate disease activity (MDA) with a relatively high size effect (Partial 2=0.323, p0.001). HDA group has a significantly higher MMP3, CRP, RF, and anti-citrullinated protein antibodies (ACPA) than the MDA group. Conclusions. The use of the NN analysis indicated that the measured biomarkers help predict the HDA state in RA patients. MMP3 and osteopontin are diagnostic biomarkers for the severity of RA disease and related to many disease-related characteristics with a sensitivity of 88.9% and specificity of 68.4%.

Recent Progress of RGD Modified Liposomes as Multistage Rocket Against Cancer

Cancer is a life-threatening disease, contributing approximately 9.4 million deaths worldwide. To address this challenge, scientific researchers have investigated molecules that could act as speed-breakers for cancer. As an abiotic drug delivery system, liposomes can hold both hydrophilic and lipophilic drugs, which promote a controlled release, accumulate in the tumor microenvironment, and achieve elongated half-life with an enhanced safety profile. To further improve the safety and impair the off-target effect, the surface of liposomes could be modified in a way that is easily identified by cancer cells, promotes uptake, and facilitates angiogenesis. Integrins are overexpressed on cancer cells, which upon activation promote downstream cell signaling and eventually activate specific pathways, promoting cell growth, proliferation, and migration. RGD peptides are easily recognized by integrin over expressed cells. Just like a multistage rocket, ligand anchored liposomes can be selectively recognized by target cells, accumulate at the specific site, and finally, release the drug in a specific and desired way. This review highlights the role of integrin in cancer development, so gain more insights into the phenomenon of tumor initiation and survival. Since RGD is recognized by the integrin family, the fate of RGD has been demonstrated after its binding with the acceptor’s family. The role of RGD based liposomes in targeting various cancer cells is also highlighted in the paper.

Dissecting the Inorganic Nanoparticle-Driven Interferences on Adhesome Dynamics

Inorganic nanoparticles have emerged as an attractive theranostic tool applied to different pathologies such as cancer. However, the increment in inorganic nanoparticle application in biomedicine has prompted the scientific community to assess their potential toxicities, often preventing them from entering clinical settings. Cytoskeleton network and the related adhesomes nest are present in most cellular processes such as proliferation, migration, and cell death. The nanoparticle treatment can interfere with the cytoskeleton and adhesome dynamics, thus inflicting cellular damage. Therefore, it is crucial dissecting the molecular mechanisms involved in nanoparticle cytotoxicity. This review will briefly address the main characteristics of different adhesion structures and focus on the most relevant effects of inorganic nanoparticles with biomedical potential on cellular adhesome dynamics. Besides, the review put into perspective the use of inorganic nanoparticles for cytoskeleton targeting or study as a versatile tool. The dissection of the molecular mechanisms involved in the nanoparticle-driven interference of adhesome dynamics will facilitate the future development of nanotheranostics targeting cytoskeleton and adhesomes to tackle several diseases, such as cancer.

Octacalcium phosphate collagen composite stimulates the expression and activity of osteogenic factors to promote bone regeneration

Objective

This study investigated the bone regenerative properties of an octacalcium phosphate collagen composite (OCP/Col) in a rat calvarial bone defect model.

Design

An OCP/Col or β‐tricalcium phosphate (β‐TCP)/Col disk was implanted into the critical‐sized calvarial defects and fixed 2 or 4 weeks later. The radiopacity of defects was examined after disk implantation by the radiographic examination and micro‐computed tomography (μ‐CT). Immunohistochemical and histochemical analyses were carried out to assess the bone matrix maturation, neovascularization, and osteoclast and osteoblast distribution in the neonatal bone.

Results

Radiographic and μ‐CT examination of the area of implanted OCP/Col indicated the newly formed bone and no difference from those of the original bone. Osteopontin, osteocalcin, Runt‐related transcription factor 2, type 1 collagen, vascular endothelial growth factor, and alkaline phosphatase or tartrate‐resistant acid phosphatase in the newly formed calvarial bone and the surrounding connective tissue were detected by immunohistochemistry and histochemistry. Biomarker expression was not significantly elevated at the defect site; the area of which was calculated by dividing the distance from the healthy bone margin or calvarium and dura mater surface. There was no difference in the expression of these biomarkers in the OCP/Col group at 2 and 4 weeks after surgery. In addition, the expression levels of all markers were higher in the OCP/Col group than in the β‐TCP/Col group at 2 and 4 weeks after surgery.

Conclusions

The OCP/Col as a bone regeneration material not only exhibits osteoconductive activity that is dependent on residual healthy bone tissue, but also has osteoinductive capacity, which promotes angiogenesis and osteogenic cell invasion from host tissue into the bone defect.

Collagen and non-collagenous proteins molecular crosstalk in the pathophysiology of osteoporosis

Collagenous and non-collagenous proteins (NCPs) in the extracellular matrix, as well as the coupling mechanisms between osteoclasts and osteoblasts, work together to ensure normal bone metabolism. Each protein plays one or more critical roles in bone metabolism, sometimes even contradictory, thus affecting the final mechanical, physical and chemical properties of bone tissue. Anomalies in the amount and structure of one or more of these proteins can cause abnormalities in bone formation and resorption, which consequently leads to malformations and defects, such as osteoporosis (OP). The connections between key proteins involved in matrix formation and resorption are far from being elucidated. In this review, we resume knowledge on the crosstalk between collagen type I and selected NCPs (Transforming Growth Factor-β, Insulin-like Growth Factor-1, Decorin, Osteonectin, Osteopontin, Bone Sialoprotein and Osteocalcin) of bone matrix, focusing on their possible involvement and role in OP. The different elements of this network can be pharmacologically targeted or used for the design/development of innovative regenerative strategies to modulate a feedback loop in bone remodelling.

[Variations on the theme of podosomes, context matters]

Podosomes are actin-based microdomains connecting the cell with its extracellular matrix. Contractile actin-myosin cables assemble them into a network that constitutes a versatile cellular superstructure. Discovered and extensively described in in vitro conditions, podosomes now appear as major actors of specific physiological processes. They share common characteristics but their morphology and their effect on cell functioning can only be apprehended in specific cellular contexts. We focus here on three cellular processes involving podosomes and discuss their properties in context.

Variations on the theme of podosomes: A matter of context

Extensive in vitro studies have described podosomes as actin-based structures at the plasma membrane, connecting the cell with its extracellular matrix and endowed with multiple capabilities. Contractile actin-myosin cables assemble them into a network that constitutes a multifaceted cellular superstructure taking different forms - with common characteristics - but manifesting different properties depending on the context of study. Their morphology and their role in cell functioning and behavior are therefore now apprehended in in vivo or in vitro situations relevant to physiological processes. We focus here on three of them, namely: macrophage migration, antigen presentation by dendritic cells and endothelial cell sprouting during angiogenesis to highlight the characteristics of podosomes and their functioning shaped by the microenvironment.

Spatial distribution of osteopontin, CD44v6 and podoplanin in the lining epithelium of odontogenic keratocyst, and their biological relevance

BACKGROUND AND AIMS

The odontogenic keratocyst (OKC) remains the most challenging jaw cyst to treat because of its locally-aggressive behaviour and high recurrence potential. Emerging evidence suggests that osteopontin, its receptors CD44v6 and integrin α_v, and podoplanin, have a role in the local invasiveness of this cyst. However the spatial distribution characteristics of these pro-invasive markers in the lining epithelium of OKC, and their association with the clinicopathologic parameters of OKC are largely unexplored. This study sought to address these issues in comparison with dentigerous cysts (DCs) and radicular cysts (RCs) and to evaluate their biological relevance.

METHODS

A sample consisting of 20 OKC cases, 10 DCs and 10 RCs was subjected to immunohistochemical staining for osteopontin, CD44v6 and integrin α_v, and podoplanin, and semiquantitative analysis was performed.

RESULTS

All factors (except integrin α_v) were detected heterogeneously in the constitutive layers of the lining epithelium in all three cyst types. Key observations were significant upregulation of CD44v6 and podoplanin in OKC compared to DCs and RCs, suggesting that these protein molecules may play crucial roles in promoting local invasiveness in OKC (P<0.05). Osteopontin underexpression and distribution patterns were indistinctive among all three cysts indicating its limited role as pro-invasive factor. Clinical parameters showed no significant correlations with all protein factors investigated.

CONCLUSIONS

Present findings suggest that an osteopontin^low CD44v6^high and podoplanin^high immunoprofile most probably represent epithelial signatures of OKC and are markers of local invasiveness in this cyst.

Topographical distribution of phosphorylation sites of phosvitins by mass spectrometry

Phosvitin, derived from the vitellogenin II gene protein, is a highly phosphorylated protein found in egg yolk. A second hypothetical protein has been predicted based on the vitellogenin I gene, but has not been defined at the protein level. Mass spectrometric analysis was used to identify the phosphopeptide sequences and the precise sites of phosphorylation of two phosvitins, phosvitin 1 and phosvitin 2 derived from vitellogenins I and II, respectively. Samples of native phosvitin were subjected to tryptic digestion followed by mass spectrometric analysis: (i) native phosvitin peptides, (ii) after treatment with NaOH, and (iii) after chemical derivatization of P-Ser/P-Thr residues by dithiothreitol under base-catalyzed conditions. A combination of these approaches led to the identification of 68 and 35 phosphopeptides with 89 (81 P-Ser and 8 P-Thr residues) and 62 (57 P-Ser and 5 P-Thr residues) phosphorylation sites of phosvitin 1 and phosvitin 2, respectively. These data for the first time documented on a large scale the major states and sites of phosphorylation of phosvitins with a total of 151 phosphorylation sites. Importantly, the present work also provided the first direct de novo protein amino-acid sequence data for phosvitin 1 protein and evidence for the full expression of vitellogenin I gene.

BIOLOGICAL SIGNIFICANCE

We have for the first time generated a large number of phosphopeptides (~100) and identified 151 phosphorylation sites of phosvitin 1 and phosvitin 2, respectively. Importantly, this study also led to the discovery of a novel phosvitin 1 and provided the first direct de novo protein amino-acid sequence data for the full expression of vitellogenin I gene. There is considerable interest in naturally occurring phosphopeptides/phosphoproteins and their application in biomedical fields and in the food industry because of their molecular characteristics and non-toxic nature, hence, our work opens new avenues to pursue such endeavors. In addition, the results provide important fundamental biologic information relevant to evolutionary developments of vertebrate animals beginning with the earliest fish, reptiles, birds and more contemporary mammals. For instance, the abundance of phosvitins with a unique degree of phosphorylation in the egg yolk of fish, reptiles, and birds suggests potential biological functions of phosvitins which are critical to the development of embryos of these distant vertebrates.

Cyclic-RGD is as effective as rhBMP-2 in anterior interbody fusion of the sheep cervical spine

STUDY DESIGN

Radiological and histological assessment of fusion status after anterior cervical discectomy and fusion (ACDF) procedure in a sheep spinal fusion model.

OBJECTIVE

To evaluate the efficacy of cyclic arginine-glycine-aspartic (cRGD) in comparison with recombinant human bone morphogenetic protein-2 (rhBMP-2) on a mineralized collagen matrix (MCM).

SUMMARY OF BACKGROUND DATA

A previous evaluation of MCM alone in comparison with autologous bone graft alone was not able to show an advantage on spinal fusion. The cRGD peptide sequence plays a major role in mediating cell adhesion. Studies have demonstrated enhances osteoblasts adhesion resulting in increased periimplant bone formation after implantcoating with cRGD. rhBMP-2 has already proven its ability to enhance spinal fusion. To date, no comparative in vivo evaluation of cRGD and rhBMP-2 in combination with a MCM for spinal fusion has been performed.

METHODS

Twenty-four sheep (N = 8/group) underwent C3-C4 fusion. Implants: group 1: titanium cage with MCM and rhBMP-2; group 2: titanium cage with MCM and cRGD; control group: titanium cage with MCM alone. After 12 weeks fusion sites were evaluated by computed tomography to assess fusion status, bone mineral density as well as bony callus volume. Furthermore, histomorphological and histomorphometrical analysis of the fusion sites were performed.

RESULTS

In comparison with the control group, cRGD, and rhBMP-2 groups showed a higher fusion rate in radiographical findings and a higher degree of interbody fusion in histomorphometrical analysis (P < 0.05). There was no significant difference in radiographical and histological parameters between the rhBMP-2 and the cRGD group. Although rhBMP-2 demonstrated ectopic prevertebral bone formations, this effect was less prominent in the cRGD group.

CONCLUSION

In this animal model the combination of cRGD and a mineralized collagen matrix showed superior fusion results in comparison with the mineralized collagen alone. Further, cRGD was comparably effective to rhBMP-2 in promoting interbody fusion by demonstrating less ectopic bone formations.

Osteoclasts and odontoclasts: signaling pathways to development and disease

Osteoclasts are cells essential for physiologic remodeling of bone and also play important physiologic and pathologic roles in the dentofacial complex. Osteoclasts and odontoclasts are necessary for tooth eruption yet result in dental compromise when associated with permanent tooth internal or external resorption. The determinants that separate their physiologic and pathologic roles are not well delineated. Clinical cases of primary eruption failure and root resorption are challenging to treat. Mineralized tissue resorbing cells undergo a fairly well characterized series of differentiation stages driven by transcriptional mediators. Signal transduction via cytokines and integrin-mediated events comprise the detailed pathways operative in osteo/odontoclastic cells and may provide insights to their targeted regulation. A better understanding of the unique aspects of osteoclastogenesis and osteo/odontoclast function will facilitate effective development of new therapeutic approaches. This review presents the clinical challenges and delves into the cellular and biochemical aspects of the unique cells responsible for resorption of mineralized tissues of the craniofacial complex.

Osteopontin regulates anabolic effect in human menopausal osteoporosis with intermittent parathyroid hormone treatment

In this pilot study, we demonstrated that women with osteopontin (OPN) over-expression show less resistance to postmenopausal osteoporosis than women with normal OPN levels. We hypothesized that the levels of plasma OPN could be used as a treatment indicator for intermittent parathyroid hormone (PTH)-treated menopausal osteoporosis. We demonstrated that plasma OPN levels could be used as a biomarker for early treatment response.

INTRODUCTION

Animal studies indicate that OPN-deficient mice are resistant to ovariectomy induced osteoporosis. Our pilot study also demonstrated women with OPN over expression may show less resistance to postmenopausal osteoporosis. The role of plasma OPN in PTH1-34-treated osteoporosis remains unclear.

METHODS

From September 2005 to September 2006, 31 menopausal women over 45 years of age with severe osteoporosis were enrolled in our study. Subjects were treated with PTH1-34 subcutaneously at a dose of 20 μg/day. Plasma OPN levels and BMD of the lumbar spine and hip were measured using ELISA and dual-energy X-ray absorptiometry at baseline, 3, 6, and 9 months. Response to the treatment was assessed by the sequential change in bone mineral density and OPN expression using a general linear mixed model.

RESULTS

The plasma OPN decreased sequentially and significantly throughout the 9-month treatment course from 20.75 ± 5.36 to 11.2 ± 4.37 ng/ml (p < 0.001). The sequential improvement in the T-score and Z-score was significant in the lumbar spine but not in the hip area. In the lumbar spine, when the plasma OPN decreased by 1 ng/ml the T-score increased by 0.0406 and the Z-score increased by 0.0572 of lumbar spine.

CONCLUSION

OPN levels are related to the anabolic effect of PTH in human postmenopausal osteoporosis. Plasma OPN levels could be used as a biomarker for early treatment response.

The isolation and characterization of glycosylated phosphoproteins from herring fish bones

Past studies of bone extracellular matrix phosphoproteins such as osteopontin and bone sialoprotein have yielded important biological information regarding their role in calcification and the regulation of cellular activity. Most of these studies have been limited to proteins extracted from mammalian and avian vertebrates and nonvertebrates. The present work describes the isolation and purification of two major highly glycosylated and phosphorylated extracellular matrix proteins of 70 and 22 kDa from herring fish bones. The 70-kDa phosphoprotein has some characteristics of osteopontin with respect to amino acid composition and susceptibility to thrombin cleavage. Unlike osteopontin, however, it was found to contain high levels of sialic acid similar to bone sialoprotein. The 22-kDa protein has very different properties such as very high content of phosphoserine (∼270 Ser(P) residues/1000 amino acid residues), Ala, and Asx residues. The N-terminal amino acid sequence analysis of both the 70-kDa (NPIMA(M)ETTS(M)DSKVNPLL) and the 22-kDa (NQDMAMEASSDPEAA) fish phosphoproteins indicate that these unique amino acid sequences are unlike any published in protein databases. An enzyme-linked immunosorbent assay revealed that the 70-kDa phosphoprotein was present principally in bone and in calcified scales, whereas the 22-kDa phosphoprotein was detected only in bone. Immunohistological analysis revealed diffusely positive immunostaining for both the 70- and 22-kDa phosphoproteins throughout the matrix of the bone. Overall, this work adds additional support to the concept that the mechanism of biological calcification has common evolutionary and fundamental bases throughout vertebrate species.

Increased serum osteopontin is a risk factor for osteoporosis in menopausal women

SUMMARY

Osteopontin (OPN)-deficient mice are resistant to ovariectomy-induced osteoporosis. Therefore, we hypothesized that women with OPN overexpression may show less resistance to postmenopausal osteoporosis. In this study, we first demonstrated that serum OPN levels could be used as a biomarker for the early diagnosis of osteoporosis in postmenopausal women.

INTRODUCTION

Animal studies indicate that OPN-deficient mice are resistant to ovariectomy-induced osteoporosis.

METHODS

From 2004 to 2006, 124 women over the age of 45 were enrolled in a menopausal group, while another 95 women, from 25 to 45 years of age with regular menstruation, were enrolled into a childbearing age group. The serum concentrations of OPN were calculated using the enzyme-link immunosorbent assay method, and bone mineral densities were determined with dual energy X-ray absorptiometry.

RESULTS

Serum OPN levels had a significant positive correlation with age (menopausal group, p < 0.0001) and a negative correlation with body weight, height, hip bone mineral density, and T-scores in the menopausal group. In contrast, there was a positive correlation with the E2 concentration and height, but there was no significant association with the above variables in the childbearing age group. Additionally, high serum OPN levels (>14.7 ng/ml) was a significant risk factor causing menopausal osteoporosis (odds ratio = 2.96, 95% confidence interval, 1.055-8.345).

CONCLUSION

Serum OPN levels could be used as a biomarker for the early diagnosis of osteoporosis in postmenopausal women.

High expression of osteopontin and CD44v6 in odontogenic keratocysts

BACKGROUND/PURPOSE

Odontogenic keratocysts (OKCs) are more aggressive and more osteolytic lesions than dentigerous cysts (DCs) and radicular cysts (RCs). Osteopontin (OPN) is related to cancer metastasis and bone destruction. Binding of OPN to its cell membrane receptors integrin alphav and CD44v6 can enhance tumor cell motility, migration, invasion and spread. This study assessed the possible contribution of OPN, integrin alphav and CD44v6 to the local aggressive behavior and osteolytic ability of OKCs.

METHODS

We used an immunohistochemical method to examine the expression of OPN, integrin alphav and CD44v6 in tissue sections of 20 OKCs, eight DCs and 10 RCs.

RESULTS

We found strong cytoplasmic OPN immunostaining in lining epithelial cells of 8 of 20 OKCs but not in any DCs and RCs. Positive OPN staining was also noted in the subepithelial connective tissue of four OKCs with intraepithelial expression of OPN. Diffuse and strong membranous integrin alphav staining was discovered in osteoclasts in all our tissue sections and in nearly all lining epithelial cells of DCs and RCs, but not in OKCs. In addition, diffuse and strong membranous CD44v6 staining was also observed in nearly all lining epithelial cells of OKCs, DCs and RCs.

CONCLUSION

Binding of OPN to osteoclast cell membrane receptor integrin alphav can activate the osteoclasts and increase their osteolytic activity. In addition, binding of OPN to OKC lining epithelial cell membrane receptor CD44v6 can enhance the motility, migration, invasion and spread of lining epithelial cells into the surrounding cancellous bone. Therefore, we suggest that the local aggressive behavior and high osteolytic ability of OKCs in the jawbone can be explained at least partially by high expression of OPN and CD44v6 in lining epithelial cells of OKCs and high expression of integrin alphav in osteoclasts.

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.

Rho-ROCK signaling differentially regulates chondrocyte spreading on fibronectin and bone sialoprotein

The mammalian growth plate is a dynamic structure rich in extracellular matrix (ECM). Interactions of growth plate chondrocytes with ECM proteins regulate cell behavior. In this study, we compared chondrocyte adhesion and spreading dynamics on fibronectin (FN) and bone sialoprotein (BSP). Chondrocyte adhesion and spreading were also compared with fibroblasts to analyze potential cell-type-specific effects. Chondrocyte adhesion to BSP is independent of posttranslational modifications but is dependent on the RGD sequence in BSP. Whereas chondrocytes and fibroblasts adhered at similar levels on FN and BSP, cells displayed more actin-dependent spread on FN despite a 16x molar excess of BSP adsorbed to plastic. To identify intracellular mediators responsible for this difference in spreading, we investigated focal adhesion kinase (FAK)-Src and Rho-Rho kinase (ROCK) signaling. Although activated FAK localized to the vertices of adhered chondrocytes, levels of FAK activation did not correlate with the extent of spreading. Furthermore, Src inhibition reduced chondrocyte spreading on both FN and BSP, suggesting that FAK-Src signaling is not responsible for less cell spreading on BSP. In contrast, inhibition of Rho and ROCK in chondrocytes increased cell spreading on BSP and membrane protrusiveness on FN but did not affect cell adhesion. In fibroblasts, Rho inhibition increased fibroblast spreading on BSP while ROCK inhibition changed membrane protrusiveness of FN and BSP. In summary, we identify a novel role for Rho-ROCK signaling in regulating chondrocyte spreading and demonstrate both cell- and matrix molecule-specific mechanisms controlling cell spreading.

Bone sialoprotein expression enhances osteoblast differentiation and matrix mineralization in vitro

Bone sialoprotein (BSP) is an acidic, noncollagenous glycoprotein abundantly expressed in mineralized tissues. Although BSP is frequently used as a marker of osteoblast differentiation, the role of the protein in osteoblast function is unclear. BSP belongs to the SIBLING (Small Integrin-binding LIgand N-linked Glycoprotein) family of RGD-containing matrix proteins, several members of which have been shown to affect cell differentiation. The normal levels of BSP expression in osteoblasts were specifically altered by CMV-mediated adenoviral overexpression in primary osteoblasts or inhibition by an RNA interference-based strategy in the MC3T3E1 cell line. Alternatively, osteoblast cultures were supplemented with recombinant BSP protein. Quantitative real-time PCR was used to monitor the mRNA levels of the osteoblast-related transcription factors Osterix and Runx2 as well as the osteoblast-specific gene osteocalcin. As markers of osteoblast differentiation, alkaline phosphatase enzyme activity, Runx2-luciferase reporter activity and calcein incorporation into mineralized cultures were also measured. The overexpression of BSP increased osteoblast-related gene expression as well as calcium incorporation and nodule formation by osteoblast cultures. Similarly, supplementation of osteoblast cultures with recombinant BSP increased several markers of osteoblast differentiation. Conversely, suppression of BSP expression by small-hairpin RNA-encoding plasmids inhibited expression of osteoblast markers and nodule formation. Overexpression of several functional-domain mutants of BSP demonstrated that increases in osteoblast-related gene expression and matrix mineralization observed in BSP overexpression models are mediated by the integrin-binding RGD motif found near the C-terminus of the protein. These results demonstrate that BSP may serve as a matrix-associated signal directly promoting osteoblast differentiation resulting in the increased production of a mineralized matrix.

Oral fluid-based biomarkers of alveolar bone loss in periodontitis

Periodontal disease is a bacteria-induced chronic inflammatory disease affecting the soft and hard supporting structures encompassing the teeth. When left untreated, the ultimate outcome is alveolar bone loss and exfoliation of the involved teeth. Traditional periodontal diagnostic methods include assessment of clinical parameters and radiographs. Though efficient, these conventional techniques are inherently limited in that only a historical perspective, not current appraisal, of disease status can be determined. Advances in the use of oral fluids as possible biological samples for objective measures of current disease state, treatment monitoring, and prognostic indicators have boosted saliva and other oral-based fluids to the forefront of technology. Oral fluids contain locally and systemically derived mediators of periodontal disease, including microbial, host-response, and bone-specific resorptive markers. Although most biomarkers in oral fluids represent inflammatory mediators, several specific collagen degradation and bone turnover-related molecules have emerged as possible measures of periodontal disease activity. Pyridinoline cross-linked carboxyterminal telopeptide (ICTP), for example, has been highly correlated with clinical features of the disease and decreases in response to intervention therapies, and has been shown to possess predictive properties for possible future disease activity. One foreseeable benefit of an oral fluid-based periodontal diagnostic would be identification of highly susceptible individuals prior to overt disease. Timely detection and diagnosis of disease may significantly affect the clinical management of periodontal patients by offering earlier, less invasive, and more cost-effective treatment therapies.

Regular expression of osteopontin in granular cell tumor: distinctive feature among Schwannian cell tumors

For further characterization of S-100 protein-positive granular cell tumor (GCT), the expression of osteopontin (OPN) in the tumor cells was compared with other types of Schwannian cell tumors. Twenty GCT, three amputation neuromas, 12 neurilemmomas (NM) and three malignant peripheral nerve sheath tumors (MPNST) were used. In addition, two granular cell epulides were employed because of histological similarity to that of GCT. Immunohistochemistry by anti-OPN-monoclonal antibody revealed OPN-immunoreactivity in tumor cells of all GCT and stromal macrophages in neurilemmomas and one MPNST, but neither in amputation neuromas nor in granular cell epulides. All nine GCT studied by in situ hybridization (ISH) and all three GCT analyzed by reverse transcription-polymerase chain reaction (RT-PCR) were positive for OPN mRNA. In contrast, seven NM and three amputation neuromas were negative for OPN mRNA by either ISH or RT-PCR, while macrophages infiltrated in these tumors had OPN mRNA consistently. Double immunostaining for OPN and CD68, a lysosome-associated glycoprotein, showed their colocalization in tumor cells of GCT, suggesting a possible degradation of OPN by lysosomes. In conclusion, GCT among Schwannian cell tumors consistently express OPN, but its biological significance requires further investigation.

Osteoclast responses to lipopolysaccharide, parathyroid hormone and bisphosphonates in neonatal murine calvaria analyzed by laser scanning confocal microscopy

Because the development and activity of osteoclasts in bone remodeling is critically dependent on cell-cell and cell-matrix interactions, we used laser confocal microscopy to study the response of osteoclasts to lipopolysaccharide (LPS; 10 microg/ml), parathyroid hormone (PTH; 10(-8) M), and bisphosphonates (BPs; 1-25 microM clodronate or 0.1-2.5 microM risedronate) in cultured neonatal calvaria. Following treatment with LPS or PTH (<48 hr), osteopontin (OPN) and the alphavbeta3 integrin were found colocalized with the actin ring in the sealing zone of actively resorbing osteoclasts. In contrast, non-resorbing osteoclasts in BP-treated cultures showed morphological abnormalities, including retraction of pseudopods and vacuolization of cytoplasm. In the combined presence of LPS and BP, bone-resorbing osteoclasts were smaller and the sealing zone diffuse, reflecting reduced actin, OPN, and beta3 integrin staining. Depth analyses of calvaria showed that the area of resorbed bone was filled with proliferating osteoblastic cells that stained for alkaline phosphatase, collagen type I, and bone sialoprotein, regardless of the presence of BPs. These studies show that confocal microscopy of neonatal calvaria in culture can be used to assess the cytological relationships between osteoclasts and osteoblastic cells in response to agents that regulate bone remodeling in situ, avoiding systemic effects that can compromise in vivo studies and artifacts associated with studies of isolated osteoclasts.

Root resorption: The possible role of extracellular matrix proteins

During maxillary permanent canine eruption, the deciduous canine root is selectively resorbed while the adjacent permanent lateral incisor root is left intact. An understanding of this selective resorption could provide insights into the cause of moderate to severe iatrogenic root resorption during orthodontic tooth movement. This study investigated the possible role of extracellular matrix (ECM) proteins in the selective resorption process. The spatial expression patterns of 2 ECM proteins, osteopontin (OPN) and bone sialoprotein (BSP), were assessed within the periodontal ligament (PDL) surrounding root surface areas of deciduous and permanent teeth. Intact and resorbed root surface areas from 14 deciduous roots and intact root surface areas from 12 permanent tooth roots were examined. In the deciduous roots, BSP and OPN were locally expressed in the cell layer adjacent to the root surface and most intensely concentrated in areas surrounding and within odontoclasts proximal to the resorptive lacunae. In contrast, BSP and OPN were expressed in a generalized pattern throughout the PDL of permanent roots. These preliminary findings suggest a differential expression of ECM proteins on deciduous versus permanent tooth roots, which could act as a signal for selective odontoclast adhesion to, and subsequent resorption of, deciduous root surfaces.

Parathyroid hormone-induced bone resorption does not occur in the absence of osteopontin

Osteopontin is an RGDS-containing protein that acts as a ligand for the alpha(v)beta(3) integrin, which is abundantly expressed in osteoclasts, cells responsible for bone resorption in osteopenic diseases such as osteoporosis and hyperparathyroidism. However, the role of osteopontin in the process of bone resorption has not yet been fully understood. Therefore, we investigated the direct function of osteopontin in bone resorption using an organ culture system. The amount of (45)Ca released from the osteopontin-deficient bones was not significantly different from the basal release from wild type bones. However, in contrast to the parathyroid hormone (PTH) enhancement of the (45)Ca release from wild type bones, PTH had no effect on (45)Ca release from organ cultures of osteopontin-deficient bones. Because PTH is located upstream of receptor activator of NF-kappaB ligand (RANKL), that directly promotes bone resorption, we also examined the effect of RANKL. Soluble RANKL with macrophage-colony stimulating factor enhanced (45)Ca release from the bones of wild type fetal mice but not from the bones of osteopontin-deficient mice. To obtain insight into the cellular mechanism underlying the phenomena observed in osteopontin-deficient bone, we investigated the number of tartrate-resistant acid phosphatase (TRAP)-positive cells in the bones subjected to PTH treatment in cultures. The number of TRAP-positive cells was increased significantly by PTH in wild type bone; however, no such PTH-induced increase in TRAP-positive cells was observed in osteopontin-deficient bones. These results indicate that the absence of osteopontin suppressed PTH-induced increase in bone resorption via preventing the increase in the number of osteoclasts in the local milieu of bone.

Osteopontin

Osteopontin (OPN) is a highly phosphorylated sialoprotein that is a prominent component of the mineralized extracellular matrices of bones and teeth. OPN is characterized by the presence of a polyaspartic acid sequence and sites of Ser/Thr phosphorylation that mediate hydroxyapatite binding, and a highly conserved RGD motif that mediates cell attachment/signaling. Expression of OPN in a variety of tissues indicates a multiplicity of functions that involve one or more of these conserved motifs. While the lack of a clear phenotype in OPN "knockout" mice has not established a definitive role for OPN in any tissue, recent studies have provided some novel and intriguing insights into the versatility of this enigmatic protein in diverse biological events, including developmental processes, wound healing, immunological responses, tumorigenesis, bone resorption, and calcification. The ability of OPN to stimulate cell activity through multiple receptors linked to several interactive signaling pathways can account for much of the functional diversity. In this review, we discuss the structural features of OPN that relate to its function in the formation, remodeling, and maintenance of bones and teeth.

Increased expression of bone sialoprotein in bone metastases compared with visceral metastases in human breast and prostate cancers

The recent demonstration that bone sialoprotein (BSP) is expressed in osteotropic cancers suggests that this bone matrix protein might be implicated in the preferential seed and growth of metastatic cells in bone. High expression of BSP in breast and prostate primary carcinomas is associated with progression and bone metastases development. The exact mechanisms by which BSP may favor bone metastases formation are not clearly established yet. Although BSP expression has been detected in breast, prostate, lung, thyroid, and neuroblastoma primary tumors, no information regarding its expression in metastases is available to date. In this study, we have examined BSP expression in 15 bone and 39 visceral metastatic lesions harvested from 8 breast cancer patients and 7 prostate cancer patients who died of disseminated disease. We were able to retrieve the primary lesions from 5 of the 8 breast cancer patients as well as from all 7 prostate cancer patients. All the primary breast tumor patients and 5 of the 7 primary prostate cancer patients expressed a detectable level of BSP. Bone metastases from all 8 breast cancer patients and from 5 out of 7 prostate cancer patients exhibited detectable levels of the protein. Metastatic cells in close contact with bone trabeculae usually were highly positive for BSP. BSP also was detected in secondary lesions developed at visceral sites including liver, thyroid, lung, and adrenal glands. However, BSP expression was significantly lower in visceral metastases than in skeletal ones (Mann-Whitney test, p < 0.05). Our data represent the first demonstration of an increased expression of BSP in bone metastases compared with nonskeletal metastases in human breast and prostate cancers and add weight to the body of evidence attributing a significant role to this protein in the genesis of bone metastases.

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.

The integrin specificity of human recombinant osteopontin

The ability of full-length human recombinant osteopontin (OPN) to support the adhesion of various alphav integrin-expressing cell lines was determined in order to characterize its integrin selectivity. The identity of this protein was assessed by cDNA sequence and mass spectroscopic analysis, and confirmed as full-length OPN. Neither the human embryonic kidney 293 cell line, which expresses the alphavbeta1 integrin, nor the human colonic adenocarcinoma HT-29 cell line, which expresses the alphavbeta5 integrin, were able to adhere to OPN; both of these cell lines are deficient in the beta3 subunit. In contrast, an alphavbeta3 integrin-expressing cell line, SK-MEL-24, was able to adhere to OPN in an arginine-glycine-aspartic acid dependent manner. In addition, this OPN-mediated cellular adhesion was completely blocked with an anti-alphavbeta3 integrin antibody (LM609), confirming that only the alphavbeta3 integrin mediated this cellular adhesion. These data demonstrate that, at least among the alphav integrins, only the alphavbeta3 is able to support cellular adhesion to osteopontin. This finding may have implications for the design of therapeutics targeting OPN-integrin interactions.

Single integrin molecule adhesion forces in intact cells measured by atomic force microscopy

Cross-talk between cells and the extracellular matrix is critically influenced by the mechanical properties of cell surface receptor-ligand interactions; these interactions are especially well defined and regulated in cells capable of dynamically modifying their matrix environment. In this study, attention was focused on osteoclasts, which are absolutely dependent on integrin extracellular matrix receptors in order to degrade bone; other bone cells, osteoblasts, were used for comparison. Integrin binding forces were measured in intact cells by atomic force microscopy (AFM) for several RGD-containing (Arg-Gly-Asp) ligands and ranged from 32 to 97 picoNewtons (pN); they were found to be cell and amino acid sequence specific, saturatable and sensitive to the pH and divalent cation composition of the cellular culture medium. In contrast to short linear RGD hexapeptides, larger peptides and proteins containing the RGD sequence, such as osteopontin (a major non-collagenous bone protein) and echistatin (a high affinity RGD sequence containing antagonist snake venom protein), showed different binding affinities. This demonstrates that the context of the RGD sequence within a protein has considerable influence upon the final binding force for receptor interaction. These data also demonstrate that AFM, as a methodological approach, can be adapted to cell biology studies wherever cell-matrix interactions play a critical role, and, moreover, may have applicability to the analysis of receptor-ligand interactions in cell membranes in general.

Granulocyte macrophage-colony stimulating factor reciprocally regulates alphav-associated integrins on murine osteoclast precursors

The integrins alphavbeta3 and alphavbeta3 are expressed reciprocally during murine osteoclastogenesis in vitro. Specifically, immature osteoclast precursors, in the form of bone marrow macrophages, contain exclusively alphavbeta5, surface expression of which declines with commitment to the osteoclast phenotype, while levels of alphavbeta3 increase concomitantly. The distinct functional significance of alphavbeta5 is underscored by the integrin's capacity, unlike alphavbeta3, to mediate both attachment and spreading on ligand, of marrow macrophages, suggesting alphavbeta3 negotiates initial recognition, by osteoclast precursors, of bone matrix. Northern analysis demonstrates changes in the two beta-subunits, and not alphav, are responsible for these alterations. Treatment of early precursors with granulocyte-macrophage colony stimulating factor (GM-CSF) leads to alterations in beta3 and beta5 mRNA and alphavbeta5 and alphavbeta3, paralleling those occurring during osteoclastogenesis. Nuclear run-on and message stability studies demonstrate that while GM-CSF treatment of precursors alters beta5 transcriptionally, the changes in beta3 arise from prolonged mRNA t1/2. Similar to GM-CSF treatment, the rate of beta5 transcription falls during authentic osteoclastogenesis. In contrast to cytokine-induced alphavbeta3, however, that attending osteoclastogenesis reflects accelerated transcription of the beta3-subunit. Thus, while GM-CSF may participate in modulation of alphavbeta5 during osteoclast differentiation, signals other than those derived from the cytokine must regulate expression of alphavbeta3.

Protein extracts of dentin affect proliferation and differentiation of osteoprogenitor cells in vitro

Proteins associated with the mineral phase of dentin are considered to have the potential to alter cell function within the local environment, during development and regeneration of tooth/periodontal tissues. Cells that may be altered include osteoblasts, ameloblasts, periodontal ligament cells, odontoblasts, and cementoblasts. However, specific factors within dentin controlling cell activity have not been elucidated. To investigate further the role of dentin proteins in regulating cell behavior, MC3T3-E1 cells, a mouse osteoprogenitor cell line, were exposed to guanidine/EDTA extracts of dentin (G/E-D) prepared from bovine teeth. Cells, with or without G/E-D (2 to 50 microg/ml), were evaluated for proliferative activity and for mRNA expression of bone-associated genes. Results indicated that G/E-D suppressed cell proliferation and caused striking morphological changes, including the conversion of cuboidal cells into fibroblastic, spindle-shaped cells. Markers of osteoblast differentiation, osteocalcin and bone sialoprotein mRNA were decreased, while osteopontin mRNA was enhanced in cells exposed to G/E-D. Since transforming growth factor beta (TGFbeta1) has been reported to influence cells in a similar fashion, G/E-D were examined for the presence of and concentration of TGFbeta using slot blot analysis and enzyme immunoassay (ELISA), respectively. These analyses demonstrated that G/E-D contained 6.6 ng/mg of TGFbeta1. Next, cells were exposed to G/E-D in conjunction with anti-TGFbeta1,2,3 antibody. When cells were exposed to antibody, G/E-D-mediated changes in morphology and gene expression were blocked. These results suggest that TGFbeta1 and perhaps other factors in dentin can regulate cell behavior and, therefore, can influence development, remodeling, and regeneration of mineralized tissues.

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.

Expression of bone microsomal casein kinase II, bone sialoprotein, and osteopontin during the repair of calvarial defects

The temporal expression of bone microsomal casein kinase II, osteopontin, bone sialoprotein, alkaline phosphatase, and the accumulation of a solid calcium-inorganic orthophosphate mineral phase, have been charted from day 2 to day 21 during the repair of calvarial defects in rats induced by the implantation of decalcified rat bone matrix. Unlike the sequence of events that occur when the same decalcified bone matrix is implanted subcutaneously or intramuscularly, in which cases the first tissue to form in response to the implant is cartilage that subsequently calcifies and is later resorbed and replaced by bone, the repair of cranial defects is quite different. In the latter case, the first cells induced are undifferentiated mesenchymal cells and early fibroblasts followed by osteoblastic direct bone formation. Somewhat later a few small islands of cartilage are formed, widely separated and spatially distinct from the newly formed bone matrix. All of the cartilage and most of the implanted decalcified bone matrix are later resorbed and replaced by new bone by day 21. This in vivo model of the repair of a bone defect by direct bone formation has provided an excellent system to follow specific biochemical and physicochemical events. The total accumulation and rate of accumulation of the mineral and the two noncollagenous phosphoproteins (bone sialoprotein and osteopontin), as well as the activities of alkaline phosphatase, and for the first time either in vivo or in cell culture, the activity of microsomal casein kinase II, the major enzyme that phosphorylates the bone phosphoproteins, have been determined as a function of healing time in vivo. The overall general pattern of accumulation of the phosphoproteins and calcium-phosphate mineral phase and their relationships are similar to those reported in osteoblast cell cultures also monitored as a function of time.

Coordinate expression of OPN and associated receptors during monocyte/macrophage differentiation of HL-60 cells

Promyelocytic leukemia HL-60 cells promoted by PMA to differentiate along the monocyte pathway adhere to tissue culture plates. To explore the regulation of adhesion molecules in cells promoted to differentiate, the expression and secretion of osteopontin (OPN) and expression of associated cell surface receptors, CD44 and integrin subunits alpha(v), beta3, beta1, were examined. Results were as follows: 1) PMA induced OPN mRNA and OPN secretion into media; 2) untreated cells expressed beta1 and CD44 mRNA, and PMA induced alpha(v), and beta3 mRNA and increased beta1 and CD44 mRNA expression; 3) PMA increased levels of alpha(v), beta3, beta1 and CD44 protein on the cell surface; and 4) retinoic acid, which promotes granulocytic differentiation of HL-60 cells, did not affect OPN, alpha(v), beta3, beta1, or CD44 mRNA or protein expression. These data suggest that induction of OPN and associated receptors may play a role during monocytic differentiation of HL-60 cells.

Integrin subunit beta3 plays a crucial role in the movement of osteoclasts from the periosteum to the bone surface

We have shown that, when mouse parietal bones were incubated in culture medium containing indomethacin, the number of tartrate-resistant acid phosphatase-positive osteoclasts (TRAP + OCs) on the bone surface was drastically reduced (down-regulation), and the number on the periosteal membrane adjacent to the resorbing surface was increased. Subsequent incubation of bones with prostaglandin E2 (PGE2) rapidly reversed these changes (up-regulation). In the work reported here, the osteoclast-associated integrin subunit beta3 was stained by immunohistochemistry. The beta3-positive osteoclast (beta3 + OC) population on freshly isolated bone was comprised of about 67% TRAP + OCs and 33% TRAP OCs. Like TRAP + OCs, beta3 + OCs were reduced in number on the surface of bones incubated with indomethacin, but, in contrast to the TRAP + OCs, beta3 + OCs were not seen on the periosteal membrane. Following up-regulation of TRAP + OCs with PGE2, large numbers of beta3 + OCs appeared on the bone surface and, again, were not seen on the periosteal membrane. Echistatin, a peptide that binds to the alphavbeta3 integrin on osteoclasts, was found to inhibit the up-regulation of TRAP + OCs in a dose-dependent manner but had no effect on the down-regulation of TRAP + OCs. Similarly, echistatin inhibited the upregulation of beta3 + OCs on the bone surface, and, under these conditions, beta3 + OCs were observed on the periosteal membrane. The addition of anti-beta3 antibody also inhibited the up-regulation of TRAP + OCs in response to PGE2. The association of beta3 protein expression with the up-regulated osteoclast and the inhibition of up-regulation by echistatin and by anti-beta3 antibody provide strong evidence that beta3 plays an essential role in the movement of osteoclasts from the membrane to the bone.

Competition for a unique response element mediates retinoic acid inhibition of vitamin D3-stimulated transcription

We have identified a novel steroid hormone response element in the avian beta3 integrin promoter. This sequence, comprising three hexameric direct repeat half-sites separated by nine and three nucleotides binds vitamin D receptor (VDR)-retinoid X receptor (RXR) and retinoic acid receptor (RAR)-RXR heterodimers. VDR-RXR binds direct repeats separated by three base pairs, and RAR-RXR recognizes half-sites separated by nine bases, whereas the central half-site interacts with both heterodimers. Retinoic acid and 1, 25-dihydroxyvitamin D3 activate both a genomic fragment including the transcriptional start site and an oligonucleotide containing the three repeats, linked to a heterologous promoter. Co-addition of the steroids produces neither synergy nor an additive effect; rather the result equals that for retinoic acid alone. Scatchard analysis demonstrates that RAR-RXR has greater affinity than VDR-RXR for the composite element. Based on these findings we propose a model in which there is specific, polarity-defined binding of VDR-RXR and RAR-RXR to three half-sites, which form two overlapping steroid response elements, with the central half-site common to both. Our results identify a novel mechanism by which one steroid hormone can modulate the activity of a second, by competing for a shared half-site in a composite response element.

Phosphorylation of purified bovine bone sialoprotein and osteopontin by protein kinases

The large number of covalently bound phosphates on the extracellular phosphoproteins osteopontin (OPN) and bone sialoprotein (BSP) have been implicated in biological functions such as mineral deposition and osteoclast binding. In the present study the state of phosphorylation of BSP and OPN was evaluated by in vitro 32P labeling using a series of protein kinases and quantification. Both the purified bovine BSP and OPN were radiolabeled by [32P]ATP and factor-independent protein kinase. Quantification of 32P radioactivity incorporated on dephosphorylated BSP and OPN provided 6.6 and 8.9 mol of phosphate incorporated/mol, respectively. Native OPN incorporated 1.07 and BSP 2.46 mol of phosphate/mol by factor-independent protein kinase. These data led to calculations that OPN and BSP, respectively, contain 7.83 and 4.14 mol of phosphate/mol in their natural state. Thrombin digests of 32P-labeled BSP showed radioactivity to be associated with fragment of approximately molecular mass values 30 kDa (N-terminal half), with no observable radioactivity associated with the 40-kDa fragment (C-terminal half). Similar experiments with 32P-labeled OPN provided two radiolabeled thrombin fragments, with molecular mass 30 kDa (N-terminal half) and 20 kDa (C-terminal half), both were radioactive. The major phosphorylation was associated with the N-terminal half containing 7.0 mol of phosphate, and 1.9 mol of phosphate were associated with the C-terminal half. Additional experiments of in vitro phosphorylation of OPN and BSP by several other known protein kinases were carried out. cAMP-dependent protein kinase showed no phosphorylation of OPN or BSP, while protein kinase C and cGMP-dependent protein kinase led to minor phosphorylation, each of the latter introduced about 1 mol of phosphate/mol of OPN and BSP molecule.