Effects of phosphates on the expression of tissue-nonspecific alkaline phosphatase gene and phosphate-regulating genes in short-term cultures of human osteosarcoma cell lines

Differing responses of osteogenic cell lines to β-glycerophosphate

Ascorbic acid (Asc), dexamethasone (Dex) and β-glycerophosphate (β-Gly) are commonly used to promote osteogenic behaviour by osteoblasts in vitro. According to the literature, several osteosarcoma cells lines appear to respond differently to the latter with regards to proliferation kinetics and osteogenic gene transcription. Unsurprisingly, these differences lead to contrasting data between publications that necessitate preliminary studies to confirm the phenotype of the chosen osteosarcoma cell line in the presence of Asc, Dex and β-Gly. The present study exposed Saos-2 cells to different combinations of Asc, Dex and β-Gly for 14 days and compared the response with immortalised human mesenchymal stromal/stem cells (MSCs). Cell numbers, cytotoxicity, mineralised matrix deposition and cell proliferation were analysed to assess osteoblast-like behaviour in the presence of Asc, Dex and β-Gly. Additionally, gene expression of runt-related transcription factor 2 (RUNX2); osteocalcin (OCN); alkaline phosphatase (ALP); phosphate regulating endopeptidase homolog X-linked (PHEX); marker of proliferation MKI67 and proliferating cell nuclear antigen (PCNA) was performed every two days during the 14-day cultures. It was found that proliferation of Saos-2 cells was significantly decreased by the presence of β-Gly which contrasted with hMSCs where no change was observed. Furthermore, unlike hMSCs, Saos-2 cells demonstrated an upregulated expression of late osteoblastic markers, OCN and PHEX that suggested β-Gly could affect later stages of osteogenic differentiation. In summary, it is important to consider that β-Gly significantly affects key cell processes of Saos-2 when using it as an osteoblast-like cell model.

Phosphorylated lipid-conjugated oligonucleotide selectively anchors on cell membranes with high alkaline phosphatase expression

Attachment of lipid tails to oligonucleotides has emerged as a powerful technology in constructing cell membrane-anchorable nucleic acid-based probes. In practice, however, conventional lipid-conjugated oligonucleotides fail to distinguish among different cell membranes. Herein, a phosphorylated lipid-conjugated oligonucleotide (DNA-lipid-P) is reported for alkaline phosphatase (ALP)-dependent cell membrane adhesion. In the absence of ALP, DNA-lipid-P with its poor hydrophobicity shows only weak interaction with cell membrane. However, in the presence of the highly expressed plasma membrane-associated ALP, DNA-lipid-P is converted to lipid-conjugated oligonucleotide (DNA-lipid) by enzymatic dephosphorylation. As a result of such conversion, the generated DNA-lipid has greater hydrophobicity than DNA-lipid-P and is thus able to insert into cell membranes in situ. Accordingly, DNA-lipid-P enables selective anchoring on cell membranes with elevated ALP level. Since elevated ALP level is a critical index of some diseases and even cancers, DNA-lipid-P holds promise for cell membrane engineering and disease diagnostics at the molecular level.

Membrane-anchored DNA probes have been used to study molecular interactions and control cell assembly, but are not selective for different cell membranes. Here the authors develop a lipid-conjugated oligonucleotide for alkaline phosphatase-dependent cell membrane anchorage and use it to distinguish different cancer cells.

Identification of co-expression modules and pathways correlated with osteosarcoma and its metastasis

BACKGROUND

Osteosarcoma is the most common bone tumor that occurs in children.

METHODS

To identify co-expression modules and pathways correlated with osteosarcoma and its clinical characteristics, we performed weighted gene co-expression network analysis (WGCNA) on RNA-seq data of osteosarcoma with 52 samples. Then we performed pathway enrichment analysis on genes from significant modules.

RESULTS

A total of 5471 genes were included in WGCNA, and 16 modules were identified. Module-trait analysis identified that a module involved in microtubule bundle formation, drug metabolism-cytochrome P450, and IL-17 signaling pathway was negatively correlated with osteosarcoma and positively correlated with metastasis; a module involved in DNA replication was positively correlated with osteosarcoma; a module involved in cell junction was positively correlated with metastasis; and a module involved in heparin binding negatively correlated with osteosarcoma. Moreover, expression levels in four of the top ten differentially expressed genes were validated in another independent dataset.

CONCLUSIONS

Our analysis might provide insight for molecular mechanisms of osteosarcoma.

lncRNA HOTAIR Inhibits Mineralization in Osteoblastic Osteosarcoma Cells by Epigenetically Repressing ALPL

HOTAIR is a lncRNA that plays critical role in gene regulation and chromatin dynamics through epigenetic mechanisms. In this work we studied the physiological role of HOTAIR during the process of mineralization using osteoblastic osteosarcoma cells focusing in ALPL (Tissue Non-Specific Alkaline Phosphatase), a pivotal gene that controls bone formation. HOTAIR knockdown resulted in upregulation of ALPL, increase of alkaline phosphatase (ALP) activity, and enhanced mineralization in osteoblastic SaOS-2 cells cultured in mineralizing medium. Luciferase assays using reporter vectors containing ALPL promoter showed that HOTAIR repression increases ALPL promoter activity. Furthermore, HOTAIR knockdown increased histone H3K4 methylation levels at ALPL promoter region, suggesting that ALPL repression by HOTAIR is regulated by epigenetic mechanisms. This work supports that physiological bone formation is epigenetically regulated by a lncRNA.

Differential expression of alkaline phosphatase gene in proliferating primary lymphocytes and malignant lymphoid cell lines

Alkaline Phosphatase (APase) activity has been shown to be enhanced specifically in mitogen stimulated B lymphocytes committed to proliferation, but not in T lymphocytes. APase gene expression was analyzed in proliferating murine and human primary lymphocytes and human malignant cell lines using reverse transcriptase and real time PCR. In mitogen stimulated murine splenic lymphocytes, enhancement of APase activity correlated well with an increase in APase gene expression. However, in mitogen stimulated murine T lymphocytes and human PBL despite a vigorous proliferative response, no increase in APase enzyme activity or gene expression was observed. A constitutive expression of APase activity concomitant with APase gene expression was observed inhuman myeloma cell line, U266 B1. However, neither enzyme activity nor gene expression of APase were observed in human T cell lymphoma, SUPT-1. The results suggest a differential expression of APase activity and its gene in proliferating primary lymphocytes of mice and humans. The specific expression of APase activity and its gene only in human myeloma cells, but not in proliferating primary B cells can be exploited as a sensitive disease marker.

Transcriptional profiling of the human fibrillin/LTBP gene family, key regulators of mesenchymal cell functions

The fibrillins and latent transforming growth factor binding proteins (LTBPs) form a superfamily of extracellular matrix (ECM) proteins characterized by the presence of a unique domain, the 8-cysteine transforming growth factor beta (TGFβ) binding domain. These proteins are involved in the structure of the extracellular matrix and controlling the bioavailability of TGFβ family members. Genes encoding these proteins show differential expression in mesenchymal cell types which synthesize the extracellular matrix. We have investigated the promoter regions of the seven gene family members using the FANTOM5 CAGE database for human. While the protein and nucleotide sequences show considerable sequence similarity, the promoter regions were quite diverse. Most genes had a single predominant transcription start site region but LTBP1 and LTBP4 had two regions initiating different transcripts. Most of the family members were expressed in a range of mesenchymal and other cell types, often associated with use of alternative promoters or transcription start sites within a promoter in different cell types. FBN3 was the lowest expressed gene, and was found only in embryonic and fetal tissues. The different promoters for one gene were more similar to each other in expression than to promoters of the other family members. Notably expression of all 22 LTBP2 promoters was tightly correlated and quite distinct from all other family members. We located candidate enhancer regions likely to be involved in expression of the genes. Each gene was associated with a unique subset of transcription factors across multiple promoters although several motifs including MAZ, SP1, GTF2I and KLF4 showed overrepresentation across the gene family. FBN1 and FBN2, which had similar expression patterns, were regulated by different transcription factors. This study highlights the role of alternative transcription start sites in regulating the tissue specificity of closely related genes and suggests that this important class of extracellular matrix proteins is subject to subtle regulatory variations that explain the differential roles of members of this gene family.

Inorganic polyphosphates: biologically active biopolymers for biomedical applications

Inorganic polyphosphate (polyP) is a widely occurring but only rarely investigated biopolymer which exists in both prokaryotic and eukaryotic organisms. Only in the last few years, this polymer has been identified to cause morphogenetic activity on cells involved in human bone formation. The calcium complex of polyP was found to display a dual effect on bone-forming osteoblasts and bone-resorbing osteoclasts. Exposure of these cells to polyP (Ca(2+) complex) elicits the expression of cytokines that promote the mineralization process by osteoblasts and suppress the differentiation of osteoclast precursor cells to the functionally active mature osteoclasts dissolving bone minerals. The effect of polyP on bone formation is associated with an increased release of the bone morphogenetic protein 2 (BMP-2), a key mediator that activates the anabolic processes leading to bone formation. In addition, polyP has been shown to act as a hemostatic regulator that displays various effects on blood coagulation and fibrinolysis and might play an important role in platelet-dependent proinflammatory and procoagulant disorders.

Inorganic polymeric phosphate/polyphosphate as an inducer of alkaline phosphatase and a modulator of intracellular Ca2+ level in osteoblasts (SaOS-2 cells) in vitro

Inorganic polymeric phosphate is a physiological polymer that accumulates in bone cells. In the present study osteoblast-like SaOS-2 cells were exposed to this polymer, complexed in a 2:1 stoichiometric ratio with Ca(2+), polyP (Ca(2+) salt). At a concentration of 100 μM, polyP (Ca(2+) salt) caused a strong increase in the activity of the alkaline phosphatase and also an induction of the steady-state expression of the gene encoding this enzyme. Comparative experiments showed that polyP (Ca(2+) salt) can efficiently replace β-glycerophosphate in the in vitro hydroxyapatite (HA) biomineralization assay. In the presence of polyP (Ca(2+) salt) the cells extensively form HA crystallites, which remain intimately associated with or covered by the plasma membrane. Only the tips of the crystallites are directly exposed to the extracellular space. Element mapping by scanning electron microscopy/energy-dispersive X-ray spectroscopy coupled to a silicon drift detector supported the finding that organic material was dispersed within the crystallites. Finally, polyP (Ca(2+) salt) was found to cause an increase in the intracellular Ca(2+) level, while polyP, as well as inorganic phosphate (P(i)) or Ca(2+) alone, had no effect at the concentrations used. These findings are compatible with the assumption that polyP (Ca(2+) salt) is locally, on the surface of the SaOS-2 cells, hydrolyzed to P(i) and Ca(2+). We conclude that the inorganic polymer polyP (Ca(2+) salt) in concert with a second inorganic, and physiologically occurring, polymer, biosilica, activates osteoblasts and impairs the maturation of osteoclasts.

Immobilization of alkaline phosphatase on microporous nanofibrous fibrin scaffolds for bone tissue engineering

Alkaline phosphatase (ALP) promotes bone formation by degrading inorganic pyrophosphate (PP(i)), an inhibitor of hydroxyapatite formation, and generating inorganic phosphate (P(i)), an inducer of hydroxyapatite formation. P(i) is a crucial molecule in differentiation and mineralization of osteoblasts. In this study, a method to immobilize ALP on fibrin scaffolds with tightly controllable pore size and pore interconnection was developed, and the biological properties of these scaffolds were characterized both in vitro and in vivo. Microporous, nanofibrous fibrin scaffolds (FS) were fabricated using a sphere-templating method. ALP was covalently immobilized on the fibrin scaffolds using 1-ethyl-3-(dimethylaminopropyl)carbodiimide hydrochloride (EDC). Scanning electron microscopic observation (SEM) showed that mineral was deposited on immobilized alkaline phosphatase fibrin scaffolds (immobilized ALP/FS) when incubated in medium supplemented with beta-glycerophosphate, suggesting that the immobilized ALP was active. Primary calvarial cells attached, spread and formed multiple layers on the surface of the scaffolds. Mineral deposition was also observed when calvarial cells were seeded on immobilized ALP/FS. Furthermore, cells seeded on immobilized ALP/FS exhibited higher osteoblast marker gene expression compared to control FS. Upon implantation in mouse calvarial defects, both the immobilized ALP/FS and FS alone treated group had higher bone volume in the defect compared to the empty defect control. Furthermore, bone formation in the immobilized ALP/FS treated group was statistically significant compared to FS alone group. However, the response was not robust.

Functional assay of the mutant tissue-nonspecific alkaline phosphatase gene using U2OS osteoblast-like cells

Tissue-nonspecific alkaline phosphatase (TNAP) plays a key role in mineralization. A defect in the TNAP gene causes hypophosphatasia, which is characteristic of systemic skeletal hypomineralization. To determine the mineralizing ability of the mutant proteins, we developed a functional assay that uses U2OS osteoblast-like cells. Expression plasmids containing TNAP mutant cDNAs were constructed and introduced into U2OS cells, which are derived from a human osteosarcoma and exhibit very low alkaline phosphatase (ALP) activity and disabled mineralization. U2OS cells, in which active TNAP cDNAs were introduced, expressed high ALP activity and mineralized their circumstance when they were cultured with beta-glycerophosphate. The ALP activity in these U2OS cells corresponded to the activity reported for COS cells in which active TNAP cDNA was introduced. An in vitro mineralization assay of U2OS cells transfected with moderate allele cDNAs showed that approximately 35% of TNAP enzymatic activity may be the threshold value for mineralization. In addition, U2OS cells transfected with wild-type TNAP and polymorphism TNAP cDNA showed PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) induction as in SaOS-2 cells. In summary, the introduction of active TNAP cDNA into U2OS cells allowed these cells to mineralize, and this technique may be a useful functional assay of TNAP mutant proteins.

Development of an ELISA method for detecting immune complexes between tissue-nonspecific alkaline phosphatase and immunoglobulin G

A convenient method for measuring immune complexes between tissue-nonspecific alkaline phosphatase (TNSALP) and immunoglobulin G (IgG) (i.e., TNSALP-IgG) would be highly useful for routine analyses. Here, we identified a surface-active agent that would dissolve membrane but not dissociate TNSALP-IgG complexes. Next, we developed an enzyme-linked immunosorbent assay (ELISA) method for detecting TNSALP-IgG complexes with two monoclonal antibodies (MoAbs): 3-29-3R was coated on assay plates and captured TNSALP-IgG from a specimen; an horseradish peroxidase (HRP)-conjugated anti-human IgG1 then reacted with captured TNSALP-IgG to form an "immunocomplex sandwich." The immunocomplex was detected via the absorbance of an HRP substrate, resulting in a semiquantitative assay. The mean absorbance of 0.195 (n=5) measured in sera from healthy donors was designated as an arbitrary unit (AU/mL) of TNSALP-IgG concentration. The ELISA values of patient sera known to contain TNSALP-IgG complexes were greater than those of normal sera (normal, 1.86 plusmn; 0.61; patient, 9.30 plusmn; 5.44), and these data were confirmed by electrophoresis. Thus, the ELISA could detect TNSALP-IgG complexes. The intraassay coefficient of variation (CV) was within 7.4% and analytical recovery was excellent. There was no significant interference from hemolytic, lipemic, or icteric serum. In summary, an ELISA using 3-29-3R MoAb and HRP-conjugated anti-human IgG1 constitutes a reliable and convenient method for the semiquantitative detection of TNSALP-IgG complexes in human serum.

Morphogenetic activity of silica and bio-silica on the expression of genes controlling biomineralization using SaOS-2 cells

In a previous study (Schröder et al., J Biomed Mater Res B Appl Biomater 75:387-392, 2005) we demonstrated that human SaOS-2 cells, when cultivated on bio-silica matrices, respond with an increased hydroxyapatite deposition. In the present contribution we investigate if silica-based components (Na-silicate, tetraethyl orthosilicate [TEOS], silica-nanoparticles) (1) change the extent of biomineralization in vitro (SaOS-2 cells) and (2) cause an alteration of the expression of the genes amelogenin, ameloblastin, and enamelin, which are characteristic for an early stage of osteogenesis. We demonstrate that the viability of SaOS-2 cells was not affected by the silica-based components. If Na-silicate or TEOS was added together with ss-glycerophosphate, an organic phosphate donor, a significant increase in biomineralization was measured. Finally, expression levels of the amelogenin, ameloblastin, and enamelin genes were determined in SaOS-2 cells during exposure to the silica-based components. After exposure for 2 days, expression levels of amelogenin and enamelin strongly increased in response to the silica-based components, while no significant change was seen for ameloblastin. In contrast, exposure of SaOS-2 cells to ss-glycerophosphate resulted in increased expression of all three genes. We conclude that the levels of the structural molecules of the enamel matrix, amelogenin and enamelin, increase in the presence of silica-based components and substantially contribute to the extent of hydroxyapatite crystallite formation. These results demonstrate that silica-based components augment hydroxyapatite deposition in vitro and suggest that enzymatically synthesized bio-silica (via silicatein) might be a promising route for tooth reconstruction in vivo.

ATP-mediated mineralization of MC3T3-E1 osteoblast cultures

While bone is hypomineralized in hypophosphatemia patients and in tissue-nonspecific alkaline phosphatase (Tnsalp)-deficient mice, the extensive mineralization that nevertheless occurs suggests involvement of other phosphatases in providing phosphate ions for mineral deposition. Although the source of phosphate liberated by these phosphatases is unknown, pyrophosphate, ATP, pyridoxal-5'-phosphate (PLP) and phoshoethanolamine (PEA) are likely candidates. In this study, we have induced mineralization of MC3T3-E1 osteoblast cultures using ATP, and have investigated potential phosphatases involved in this mineralization process. MC3T3-E1 osteoblasts were cultured for 12 days and treated either with beta-glycerophosphate (betaGP) or ATP. Matrix and mineral deposition was examined by biochemical, cytochemical, ultrastructural and X-ray microanalytical methods. ATP added at levels of 4-5 mM resulted in mineral deposition similar to that following conventional treatment with betaGP. Collagen levels were similarly normal in ATP-mineralized cultures and transmission electron microscopy and X-ray microanalysis confirmed hydroxyapatite mineral deposition along the collagen fibrils in the ECM. Phosphate release from 4 mM ATP into the medium was rapid and resulted in approximately twice the phosphate levels than after release from 10 mM betaGP. ATP treatment did not affect mineralization by altering the expression of mineral-regulating genes such as Enpp1, Ank, and Mgp, nor phosphatase genes indicating that ATP induces mineralization by serving as a phosphate source for mineral deposition. Levamisole, an inhibitor of TNSALP, completely blocked mineralization in betaGP-treated cultures, but had minor effects on ATP-mediated mineralization, indicating that other phosphatases such as plasma membrane Ca2+ transport ATPase 1 (PMCA1) and transglutaminase 2 (TG2) are contributing to ATP hydrolysis. To examine their involvement in ATP-mediated mineralization, the inhibitors cystamine (TG2 inhibitor) and ortho-vanadate (PMCA inhibitor) were added to the cultures - both inhibitors significantly reduced mineralization whereas suppression of the phosphate release by ortho-vanadate was minor comparing to other two inhibitors. The contribution of PMCA1 to mineralization may occur through pumping of calcium towards calcification sites and TG2 can likely act as an ATPase in the ECM. Unlike the GTPase activity of TG2, its ATPase function was resistant to calcium, demonstrating the potential for participation in ATP hydrolysis and mineral deposition within the ECM at elevated calcium concentrations.

Pyrophosphate inhibits mineralization of osteoblast cultures by binding to mineral, up-regulating osteopontin, and inhibiting alkaline phosphatase activity

Inorganic pyrophosphate (PP(i)) produced by cells inhibits mineralization by binding to crystals. Its ubiquitous presence is thought to prevent "soft" tissues from mineralizing, whereas its degradation to P(i) in bones and teeth by tissue-nonspecific alkaline phosphatase (Tnap, Tnsalp, Alpl, Akp2) may facilitate crystal growth. Whereas the crystal binding properties of PP(i) are largely understood, less is known about its effects on osteoblast activity. We have used MC3T3-E1 osteoblast cultures to investigate the effect of PP(i) on osteoblast function and matrix mineralization. Mineralization in the cultures was dose-dependently inhibited by PP(i). This inhibition could be reversed by Tnap, but not if PP(i) was bound to mineral. PP(i) also led to increased levels of osteopontin (Opn) induced via the Erk1/2 and p38 MAPK signaling pathways. Opn regulation by PP(i) was also insensitive to foscarnet (an inhibitor of phosphate uptake) and levamisole (an inhibitor of Tnap enzymatic activity), suggesting that increased Opn levels did not result from changes in phosphate. Exogenous OPN inhibited mineralization, but dephosphorylation by Tnap reversed this effect, suggesting that OPN inhibits mineralization via its negatively charged phosphate residues and that like PP(i), hydrolysis by Tnap reduces its mineral inhibiting potency. Using enzyme kinetic studies, we have shown that PP(i) inhibits Tnap-mediated P(i) release from beta-glycerophosphate (a commonly used source of organic phosphate for culture mineralization studies) through a mixed type of inhibition. In summary, PP(i) prevents mineralization in MC3T3-E1 osteoblast cultures by at least three different mechanisms that include direct binding to growing crystals, induction of Opn expression, and inhibition of Tnap activity.