Estrogen receptor-related receptor alpha (ERRalpha) regulates osteopontin expression through a non-canonical ERRalpha response element in a cell context-dependent manner

We previously demonstrated that the orphan nuclear receptor, estrogen receptor-related receptor alpha (ERRalpha) is highly expressed in osteoblasts and osteoclasts, regulates osteogenesis and expression of osteoblast-associated markers in the rat calvaria cell differentiation system, and is dysregulated in the rat ovariectomy model of postmenopausal osteoporosis. There are conflicting published data on the transcriptional regulation by ERRalpha of the gene for osteopontin (OPN), an extracellular matrix protein required in bone remodeling, and a potential direct target mediating ERRalpha effects in bone. We therefore readdressed OPN gene regulation by ERRalpha in both osteoblastic (rat osteosarcoma ROS17/2.8 cells) and non-osteoblastic (HeLa) cell lines using a mouse proximal 2 kb OPN promoter fragment. A minimal OPN promoter fragment spanning from -56 to +9 bp is activated in HeLa cells but repressed it in ROS17/2.8 cells. Adenine scanning mutagenesis revealed the presence of a non-canonical ERRalpha response element in this minimal promoter. Surprisingly, prototypical inactivating mutations in the activation function 2 (AF2) domain or a naturally occurring allelic variant of ERRalpha (ERRalphaH408) were all better activators than wild-type ERRalpha in HeLa cells, activities that were generally paralleled by repression in ROS17/2.8 cells. Finally, we found that the N-terminus of ERRalpha harbors a repressor domain that acts in a cell context-dependent manner. We conclude that OPN is an ERRalpha target gene whose promoter is regulated by ERRalpha in a cell context-dependent manner and that a predicted silencing mutation in AF2 or a more flexible helix 12 increases ERRalpha transcriptional activity, effects with implications for ERRalpha as a therapeutic target in bone.

Osteopontin

Osteopontin (OPN) is a multifunctional molecule highly expressed in chronic inflammatory and autoimmune diseases, and it is specifically localized in and around inflammatory cells. OPN is a secreted adhesive molecule, and it is thought to aid in the recruitment of monocytes-macrophages and to regulate cytokine production in macrophages, dendritic cells, and T-cells. OPN has been classified as T-helper 1 cytokine and thus believed to exacerbate inflammation in several chronic inflammatory diseases, including atherosclerosis. Besides proinflammatory functions, physiologically OPN is a potent inhibitor of mineralization, it prevents ectopic calcium deposits and is a potent inducible inhibitor of vascular calcification. Clinically, OPN plasma levels have been found associated with various inflammatory diseases, including cardiovascular burden. It is thus imperative to dissect the OPN proinflammatory and anticalcific functions. OPN recruitment functions of inflammatory cells are thought to be mediated through its adhesive domains, especially the arginine-glycine-aspartate (RGD) sequence that interacts with several integrin heterodimers. However, the integrin receptors and intracellular pathways mediating OPN effects on immune cells are not well established. Furthermore, several studies show that OPN is cleaved by at least 2 classes of proteases: thrombin and matrix-metalloproteases (MMPs). Most importantly, at least in vitro , fragments generated by cleavage not only maintain OPN adhesive functions but also expose new active domains that may impart new activities. The role for OPN proteolytic fragments in vivo is almost completely unexplored. We believe that further knowledge of the effects of OPN fragments on cell responses might help in designing therapeutics targeting inflammatory and cardiovascular diseases.

Role of osteopontin in regulating hepatic inflammatory responses and toxic liver injury

Osteopontin (OPN) produced by cells of the immune system, epithelial tissue, smooth muscle cells, osteoblasts and tumor cells has been implicated in various pathophysiological functions such as cell binding, spreading and migration, and tumor metastasis. OPN is known to bind to integrins expressed on macrophages through the arginine-glycine-aspartic acid (RGD) motif and promote migration of cells resulting in granuloma. In the liver, it has been reported that hepatic Kupffer cells secrete OPN facilitating macrophage infiltration in necrotic areas following carbon tetrachloride liver toxicity. Recent work has underlined the importance of OPN as a pivotal cytokine/chemokine in the generated hepatic neutrophil response during early phase alcoholic liver injury. Increased hepatobiliary OPN expression correlated well with higher neutrophil infiltration in a rat model of alcoholic steatohepatitis. In the same model of alcoholic steatohepatitis, higher hepatic expression of OPN in females was attributed to the higher neutrophil infiltration and consequent higher female sensitivity to liver damage. OPN as a potential biomarker for inflammatory liver disease has also been recently assessed. This review will focus on studies demonstrating the role of OPN in mediating hepatic inflammation (neutrophils, monocytes/macrophages and lymphocytes) and the ensuing liver toxicity.

Pathophysiological role of osteopontin in hepatic inflammation, toxicity, and cancer

Osteopontin (OPN) is a highly modified integrin-binding extracellular matrix glycophosphoprotein produced by cells of the immune system, epithelial tissue, smooth muscle cells, osteoblasts, and tumor cells. Extensive research has elucidated the pivotal role of OPN in cell signaling that controls inflammation, tumor progression, and metastasis. OPN interaction with the integrin receptors expressed on inflammatory cells through its arginine-glycine-aspartate (RGD) and non-RGD motifs promote migration and adhesion of cells. In the liver, it has been reported that hepatic Kupffer cells secrete OPN facilitating macrophage infiltration into necrotic areas following carbon tetrachloride liver toxicity. Recent work has highlighted the role of OPN in inflammatory liver diseases such as alcoholic and nonalcoholic liver disease and T-cell-mediated hepatitis. The role of OPN in hepatocellular carcinoma (HCC) has also generated significant interest, especially with regards to its role as a prognostic factor. OPN therefore appears to play an important role during liver inflammation and cancer. In this review we will present data to demonstrate the key role played by OPN in mediating hepatic inflammation (neutrophils, monocytes/macrophages, and lymphocytes) and its role in HCC. Greater understanding of the pathophysiologic role of OPN in hepatic inflammation and cancer may enable development of novel inflammation and cancer treatment strategies.

Identification of an osteopontin-like protein in fish associated with mineral formation

Fish has been recently recognized as a suitable vertebrate model and represents a promising alternative to mammals for studying mechanisms of tissue mineralization and unravelling specific questions related to vertebrate bone formation. The recently developed Sparus aurata (gilthead seabream) osteoblast-like cell line VSa16 was used to construct a cDNA subtractive library aimed at the identification of genes associated with fish tissue mineralization. Suppression subtractive hybridization, combined with mirror orientation selection, identified 194 cDNA clones representing 20 different genes up-regulated during the mineralization of the VSa16 extracellular matrix. One of these genes accounted for 69% of the total number of clones obtained and was later identified as theS. aurata osteopontin-like gene. The 2138-bp full-length S. aurata osteopontin-like cDNA was shown to encode a 374 amino-acid protein containing domains and motifs characteristic of osteopontins, such as an integrin receptor-binding RGD motif, a negatively charged domain and numerous post-translational modifications (e.g. phosphorylations and glycosylations). The common origin of mammalian osteopontin and fish osteopontin-like proteins was indicated through an in silico analysis of available sequences showing similar gene and protein structures and was further demonstrated by their specific expression in mineralized tissues and cell cultures. Accordingly, and given its proven association with mineral formation and its characteristic protein domains, we propose that the fish osteopontin-like protein may play a role in hard tissue mineralization, in a manner similar to osteopontin in higher vertebrates.

Nanoscale ion mediated networks in bone: osteopontin can repeatedly dissipate large amounts of energy

In the nanocomposite bone, inorganic material is combined with several types of organic molecules, and these complexes have been proposed to increase the bone strength. Here we report on a mechanism of how one of these components, human osteopontin, forms large mechanical networks that can repeatedly dissipate energy through work against entropy by breaking sacrificial bonds and stretching hidden length. The behavior of these in vitro networks is similar to that of organic components in bone, acting as an adhesive layer in between mineralized fibrils.

Effect of chemically modified titanium surfaces on protein adsorption and osteoblast precursor cell behavior

PURPOSE

To investigate the effects of different chemically modified titanium surfaces on protein adsorption and the osteoblastic differentiation of human embryonic palatal mesenchymal (HEPM) cells.

MATERIALS AND METHODS

Three different surfaces were evaluated. The first, a machined surface (Ti-M), was considered a control. The second surface was acid etched (Ti-AE). The third surface was prepared by exposing the Ti-AE samples to sodium hydroxide (NaOH) solution (Ti-AAE). The surface characteristics of chemically modified titanium were investigated by means of scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and profilometry. To evaluate the production of biomarkers, commercial kits were utilized.

RESULTS

Surface composition and morphology affected the kinetics of protein adsorption. Ti-AE surfaces manifested a greater affinity for fibronectin adsorption compared to Ti-M or Ti-AAE surfaces. It was observed that Ti-AE and Ti-AAE surfaces promoted significantly greater cell attachment compared to Ti-M surfaces. Statistically significant differences were also observed in the expression of alkaline phosphatase (ALP) activity, osteocalcin, and osteopontin on all 3 titanium surfaces. ALP activity and osteocalcin production up to day 12 suggested that differentiation of the cells into osteoblasts had occurred and that cells were expressing a bone-forming phenotype.

CONCLUSIONS

It was thus concluded from this study that surface morphology and composition play a critical role in enhancing HEPM cell proliferation and differentiation into osteoblast cells.

Assembly of collagen-binding peptide with collagen as a bioactive scaffold for osteogenesis in vitro and in vivo

Bioactive scaffolds inducing cell adhesion, differentiation have been premise for optimal formation of target tissue. Collagen has been employed as a tissue regenerative scaffold especially for bone regeneration and has been chemically surface-modified to present bioactivity. Herein, we show that peptide, denoted as collagen-binding motif (CBM, GLRSKSKKFRRPDIQYPDATDEDITSHM) identified from osteopontin (OPN) protein, was able to specifically bind collagen without chemical conjugation, while presenting apatite forming capability in vitro and in vivo. Collagen surface alone was not able to induce noticeable apatite nucleation however, mineralization was evident when assembled with CBM peptide, implying that the collagen-CBM assembly played a pivotal role in biomineralization. In vivo result further demonstrated that the CBM peptide in complex with material was able to induce bone formation by helping mineralization in the bone defect. Taken together, the CBM peptide herein and its assembly with collagen can be applied as an inducer of biomineralization as well as a bioactive scaffold for bone regeneration.

Specific adsorption of osteopontin and synthetic polypeptides to calcium oxalate monohydrate crystals

Protein-crystal interactions are known to be important in biomineralization. To study the physicochemical basis of such interactions, we have developed a technique that combines confocal microscopy of crystals with fluorescence imaging of proteins. In this study, osteopontin (OPN), a protein abundant in urine, was labeled with the fluorescent dye AlexaFluor-488 and added to crystals of calcium oxalate monohydrate (COM), the major constituent of kidney stones. In five to seven optical sections along the z axis, scanning confocal microscopy was used to visualize COM crystals and fluorescence imaging to map OPN adsorbed to the crystals. To quantify the relative adsorption to different crystal faces, fluorescence intensity was measured around the perimeter of the crystal in several sections. Using this method, it was shown that OPN adsorbs with high specificity to the edges between {100} and {121} faces of COM and much less so to {100}, {121}, or {010} faces. By contrast, poly-L-aspartic acid adsorbs preferentially to {121} faces, whereas poly-L-glutamic acid adsorbs to all faces approximately equally. Growth of COM in the presence of rat bone OPN results in dumbbell-shaped crystals. We hypothesize that the edge-specific adsorption of OPN may be responsible for the dumbbell morphology of COM crystals found in human urine.

Cell type-specific post-translational modifications of mouse osteopontin are associated with different adhesive properties

Osteopontin (OPN) is a highly modified integrin-binding protein found in all body fluids. Expression of OPN is strongly correlated with poor prognosis in many different human cancers, suggesting an important but poorly understood role for this protein in tumorigenesis and metastasis. The protein exists in a number of different isoforms differing in the degree of post-translational modifications that are likely to exhibit different functional properties. This study examines for the first time the post-translational modifications of OPN from transformed cells and the effects of these modifications on cell biology. We have characterized the complete phosphorylation and glycosylation patterns of OPN expressed by murine ras-transformed fibroblasts (FbOPN) and differentiating osteoblasts (ObOPN) by a combination of mass spectrometric analyses and Edman degradation. Mass spectrometric analysis showed masses of 34.9 and 35.9 kDa for FbOPN and ObOPN, respectively. Enzymatic dephosphorylation, sequence, and mass analyses demonstrated that FbOPN contains approximately four phosphate groups distributed over 16 potential phosphorylation sites, whereas ObOPN contains approximately 21 phosphate groups distributed over 27 sites. Five residues are O-glycosylated in both isoforms. These residues are fully modified in FbOPN, whereas one site is partially glycosylated in ObOPN. Although both forms of OPN mediated robust integrin-mediated adhesion of mouse ras-transformed fibroblasts, the less phosphorylated FbOPN mediated binding of MDA-MD-435 human tumor cells almost 6-fold more than the heavy phosphorylated ObOPN. These results strongly support the hypothesis that the degree of phosphorylation of OPN produced by different cell types can regulate its function.

Injectable gel with synthetic collagen-binding peptide for enhanced osteogenesis in vitro and in vivo

A synthetic peptide denoted as collagen-binding motif (CBM) was identified from osteopontin (OPN), a multisubunit extracellular matrix (ECM) protein, by enzymatic digestion with chymotrypsin. The aim of this study was to examine the feasibility of identified CBM peptide as an active component of gel type scaffold material in osteogenesis. The binding of CBM peptide to collagen was specific and presented high affinity. Cell adhesion and growth on CBM peptide-immobilized gel were significantly increased as compared with those on gel with control peptide or without peptide. The CBM peptide-immobilized gel increased osteoblastic differentiation, followed by marked bone formation in the rabbit calvarial defect sites at 4 weeks. Taken together, the injectable gel with synthetic CBM peptide has a potential to induce osteogenesis in vitro and in vivo, suggesting its clinical application in bone regeneration procedure.

Controlling the orientation of bone osteopontin via its specific binding with collagen I to modulate osteoblast adhesion

Osteopontin (OPN) is an important matricellular protein that modulates cell functions. It is potentially an excellent surface-coating component for engineered biomaterials. It is believed that in its preferred orientation and conformation on a surface, the functional domains of OPN such as the arginine-glycine-aspartic acid (RGD) motif will be presented to cells to the greatest extent. Previously, the authors demonstrated that OPN orientation could be modulated by surface charge. In this work, the authors attempt to control the orientation/conformation of bone OPN via its specific interactions with type I collagen. Surface plasmon resonance was used to confirm the specific binding between bone OPN and collagen I. A radiolabeled OPN adsorption assay was used to determine the amount of adsorbed OPN on tissue culture polystyrene (TCPS) surfaces with or without collagen I as an interlayer. An in vitro cell adhesion assay using osteoblast MC3T3-E1 was performed to compare the functionality of collagen-bound OPN and adsorbed OPN on TCPS. With the same amount of OPN on the surfaces, the number of cells adhered to collagen-bound OPN is significantly higher than to OPN alone on TCPS. A cell inhibition assay using soluble GRGDSP peptides showed that a higher GRGDSP concentration was needed to completely block osteoblast adhesion to collagen-bound OPN than to OPN directly on TCPS. Enhanced cell adhesion and higher blocking peptide concentration suggest that collagen-bound bone OPN has a preferable orientation/conformation for cell adhesion compared with OPN alone on TCPS. Thus, the specific binding of OPN to collagen I may naturally orient OPN, thus influencing osteoblast adhesion.

Synthetic Osteopontin-derived Peptide SVVYGLR can Induce Neovascularization in Artificial Bone Marrow Scaffold Biomaterials

We have previously reported that an osteopontin-derived SVVYGLR peptide exhibited potent angiogenic activity in vitro and in vivo. In the present study, the focus points were on the in vitro effect of SVVYGLR on bone marrow stromal cell proliferation, as well as its in vivo effect on bone tissue formation when grafts made of CO3Ap-collagen sponge- as a scaffold biomaterial containing the SVVYGLR motif - were implanted. SVVYGLR peptide promoted bone marrow stromal cell proliferation. When a CO3Ap-collagen sponge containing SVVYGLR peptide was implanted as a graft into a tissue defect created in rat tibia, the migration of numerous vascular endothelial cells - as well as prominent angiogenesis - inside the graft could be detected after one week. These results thus suggested that our scaffold biomaterials including the peptide could be useful for bone tissue regeneration.

Osteopontin: a novel inflammatory mediator of cardiovascular disease

Osteopontin, also called cytokine Eta-1, is a multifunctional protein containing Arg-Gly-Asp-Ser (RGDS) cell-binding sequence. It interacts with alpha(v)beta1, alpha(v)beta3 and alpha(v)beta5 integrins and CD44 receptors. OPN is suggested to play a role during inflammation via the recruitment and retention of macrophages and T-cells to inflamed sites. OPN regulates the production of inflammatory cytokines and nitric oxide in macrophages. In this review, we will discuss diverse roles of OPN related to cardiovascular diseases, including atherosclerosis, valvular stenosis, hypertrophy, myocardial infarction and heart failure.

Molecular modulation of calcium oxalate crystallization

Calcium oxalate monohydrate (COM) is the primary constituent of the majority of renal stones. Osteopontin (OPN), an aspartic acid-rich urinary protein, and citrate, a much smaller molecule, are potent inhibitors of COM crystallization at levels present in normal urine. Current concepts of the role of site-specific interactions in crystallization derived from studies of biomineralization are reviewed to provide a context for understanding modulation of COM growth at a molecular level. Results from in situ atomic force microscopy (AFM) analyses of the effects of citrate and OPN on growth verified the critical role of site-specific interactions between these growth modulators and individual steps on COM crystal surfaces. Molecular modeling investigations of interactions of citrate with steps and faces on COM crystal surfaces provided links between the stereochemistry of interaction and the binding energy levels that underlie mechanisms of growth modification and changes in overall crystal morphology. The combination of in situ AFM and molecular modeling provides new knowledge that will aid rationale design of therapeutic agents for inhibition of stone formation.

The Multifaceted Roles of Osteopontin in Cell Signaling, Tumor Progression and Angiogenesis

Osteopontin (OPN) is a chemokine like phosphorylated glycoprotein that plays important role in cancer progression. Extensive research from various laboratories has demonstrated the likely role of OPN in regulating the cell signaling that ultimately controls tumor growth and metastasis. Several earlier reports indicated that OPN is associated with various cancers; but its functional role in carcinogenesis is still not well defined. Besides the role of OPN in tumor biology, several studies have demonstrated the pathophysiological role of OPN in diverse biological events. This review will focus on recent advances in understanding the molecular mechanism by which OPN regulates a series of signaling cascades through activation of various kinases and transcription factors that ultimately control the expression of downstream effector genes, which contribute to tumor progression and angiogenesis in vitro and animal models. We will also provide evidences that suggest the enhanced expression of OPN is not only associated with several tumor types, but its level of expression is directly correlated to various stages of the clinical specimens of breast and prostate cancers. These studies may be useful for identifying novel OPN-based therapeutic approach for the treatment of cancer.

The regulation and role of osteopontin in malignant transformation and cancer

Osteopontin (OPN) is a predominantly secreted extracellular matrix glycophosphoprotein which binds to alpha v-containing integrins and has an important role in malignant cell attachment and invasion. High OPN expression in the primary tumor is associated with early metastasis and poor outcome in human breast and other cancers. Forced OPN overexpression in benign cells may induce neoplastic-like cell behaviour including increased attachment and invasion in vitro as well as the ability to metastasize in vivo. Conversely, OPN inhibition by antisense cDNA impedes cell growth and tumor forming capacity. OPN is not mutationally activated in cancer but its expression is regulated by Wnt/Tcf signaling, steroid receptors, growth factors, ras, Ets and AP-1 transcription factors. Presumably these factors are implicated in induction of OPN overexpression in cancer. Greater understanding of the role of OPN in neoplastic change and its transcriptional regulation may enable development of novel cancer treatment strategies.

Osteopontin and cardiovascular system

A matricellular protein, osteopontin (OPN), is expressed in response to mechanical stress and similar stimuli in the heart, integrates the inter-ECM signal transduction network of component cells, and maintains efficient contractility through quantitative and qualitative control of extracellular matrix (ECM) proteins. In particular, OPN is re-expressed in the process of tissue damage; combines with other cell growth factors, cytokines, chemokines, and proteases as a cytokine itself or as an adhesion molecule; and controls the differentiation and growth of cells involved in restoration of tissues by controlling inter-cellular signal transduction and production of ECM proteins through regulation of expression levels and activity. A study using mice lacking a functional OPN gene indicated that tissue restoration fails and collagen deposition is inhibited through matrix metalloproteinases (MMPs) in mice lacking OPN. Thus, while OPN accelerates the cardiovascular remodeling process, it also regulates the balance of various inter-cellular activities. In addition, OPN not only promotes arteriosclerosis but is also closely associated with angiogenesis. With the roles of OPN expected to be clinically elucidated, the clinical use of OPN for control of cardiovascular remodeling may be feasible. Points (1) Osteopontin (OPN) efficiently propagates contraction in the heart as a matricellular protein and thereby controls ECM proteins both quantitatively and qualitatively. (2) The quantitative and qualitative control of ECM proteins is involved in interaction with OPN receptors including those of the integrin family, CD44, and others. (3) OPN promotes myocardial remodeling through TGFbeta and MMPs. (4) OPN not only promotes arteriosclerosis but is also closely associated with arteriosteogenesis. (5) In animals lacking OPN, tissue remodeling process is inhibited, especially in terms of fibrosis after myocardial infarction. (6) While the significance of OPN as an immune system molecule is still unclear in detail, the significance of OPN in the regenerative immune system has begun to be determined.