Expression of rat bone sialoprotein promoter in transgenic mice

Overexpression of MiR-335-5p Promotes Bone Formation and Regeneration in Mice

MicroRNAs (miRNAs) and the Wnt signaling pathway play critical roles in regulating bone development and homeostasis. Our previous study revealed high expression of miR-335-5p in osteoblasts and hypertrophic chondrocytes in mouse embryos and the ability of miR-335-5p to promote osteogenic differentiation by downregulating Wnt antagonist Dickkopf-1 (DKK1). The purpose of this study was to investigate the effects of miR-335-5p constitutive overexpression on bone formation and regeneration in vivo. To that end, we generated a transgenic mouse line specifically overexpressing miR-335-5p in osteoblasts lineage by the osterix promoter and characterized its bone phenotype. Bone histomorphometry and μCT analysis revealed higher bone mass and increased parameters of bone formation in transgenic mice than in wild-type littermates. Increased bone mass in transgenic mice bones also correlated with enhanced expression of osteogenic differentiation markers. Upon osteogenic induction, bone marrow stromal cells (BMSCs) isolated from transgenic mice displayed higher mRNA expression of osteogenic markers than wild-type mice BMSCs cultures. Protein expression of Runx2 and Osx was also upregulated in BMSC cultures of transgenic mice upon osteogenic induction, whereas that of DKK1 was downregulated. Most important, BMSCs from transgenic mice were able to repair craniofacial bone defects as shown by μCT analysis, H&E staining, and osteocalcin (OCN) immunohistochemistry of newly formed bone in defects treated with BMSCs. Taken together, our results demonstrate constitutive overexpression of miR-335-5p driven by an osterix promoter in the osteoblast lineage induces osteogenic differentiation and bone formation in mice and support the potential application of miR-335-5p-modified BMSCs in craniofacial bone regeneration. © 2017 American Society for Bone and Mineral Research.

p38 regulates expression of osteoblast-specific genes by phosphorylation of osterix

Osterix, a zinc finger transcription factor, is specifically expressed in osteoblasts and osteocytes of all developing bones. Because no bone formation occurs in Osx-null mice, Osterix is thought to be an essential regulator of osteoblast differentiation. We report that, in several mesenchymal and osteoblastic cell types, BMP-2 induces an increase in expression of the two isoforms of Osterix arising from two alternative promoters. We identified a consensus Sp1 sequence (GGGCGG) as Osterix binding regions in the fibromodulin and the bone sialoprotein promoters in vitro and in vivo. Furthermore, we show that Osterix is a novel substrate for p38 MAPK in vitro and in vivo and that Ser-73 and Ser-77 are the regulatory sites phosphorylated by p38. Our data also demonstrate that Osterix is able to increase recruitment of p300 and Brg1 to the promoters of its target genes fibromodulin and bone sialoprotein in vivo and that it directly associates with these cofactors through protein-protein interactions. Phosphorylation of Osterix at Ser-73/77 increased its ability to recruit p300 and SWI/SNF to either fibromodulin or bone sialoprotein promoters. We therefore propose that Osterix binds to Sp1 sequences on target gene promoters and that its phosphorylation by p38 enhances recruitment of coactivators to form transcriptionally active complexes.

Conditionally replicating adenovirus therapy utilizing bone sialoprotein promoter (Ad-BSP-E1a) in an in vivo study of treating androgen-independent intraosseous prostate cancer

BACKGROUND

Adenoviral based gene therapy has been used in clinical trials in control of advanced prostate cancer. In this study, a promising conditionally replicating adenovirus (CRAd) driven by a tissue specific bone sialoprotein promoter in controlling prostate cancer both in vitro and in vivo is demonstrated.

METHODS

C4-2B, an androgen-independent prostate cancer cell line, was treated with PBS, Ad-BSP-TK, or the Ad-BSP-E1a in vitro, and in subcutaneous and intraosseous xenographs. Cell proliferation, PSA level in condition medium, tumor volume, and/or serum PSA were followed.

RESULTS

The growth of C4-2B and the PSA production was dramatically suppressed by Ad-BSP-E1a at very low dosage (0.3 MOI) compared with PBS and Ad-BSP-TK treatment in vitro. In the subcutaneous model, the tumor volume was significantly lower statistically in the Ad-BSP-E1a treated group than the Ad-BSP-TK control group (P = 0.02). In the intraosseous model, the mice treated in the Ad-BSP-E1a treatment group demonstrated a significant lower PSA compared to that in the control group (P < 0.01) at week 8 and week 16 post-treatment.

CONCLUSIONS

The CRAd Ad-BSP-E1a revealed potential in treating prostate cancer in this model system. Using viral or none-viral mediated gene therapy to treat prostate carcinoma continues to be a potential avenue to treat afflicted men with prostate cancer.

Applications of transgenics in studies of bone sialoprotein

Bone sialoprotein (BSP) is a major non-collagenous protein in mineralizing connective tissues such as dentin, cementum and calcified cartilage tissues. As a member of the Small Integrin-Binding Ligand, N-linked Glycoprotein (SIBLING) gene family of glycoproteins, BSP is involved in regulating hydroxyapatite crystal formation in bones and teeth, and has long been used as a marker gene for osteogenic differentiation. In the most recent decade, new discoveries in BSP gene expression and regulation, bone remodeling, bone metastasis, and bone tissue engineering have been achieved with the help of transgenic mice. In this review, we discuss these new discoveries obtained from the literatures and from our own laboratory, which were derived from the use of transgenic mouse mutants related to BSP gene or its promoter activity.

Targeted overexpression of BSP in osteoclasts promotes bone metastasis of breast cancer cells

Bone is one of the most common sites of breast cancer metastasis while bone sialoprotein (BSP) is thought to play an important role in bone metastasis of malignant tumors. The objective of this study is to determine the role of BSP overexpression in osteolytic metastasis using two homozygous transgenic mouse lines in which BSP expression is elevated either in all the tissues (CMV-BSP mice) or only in the osteoclasts (CtpsK-BSP mice). The results showed that skeletal as well as systemic metastases of 4T1 murine breast cancer cells were dramatically increased in CMV-BSP mice. In CtpsK-BSP mice, it was found that targeted BSP overexpression in osteoclasts promoted in vitro osteoclastogenesis and activated osteoclastic differentiation markers such as Cathepsin K, TRAP and NFAT2. MicroCT scan demonstrated that CtpsK/BSP mice had reduced trabecular bone volume and bone mineral density (BMD). The real-time IVIS Imaging System showed that targeted BSP overexpression in osteoclasts promoted bone metastasis of breast cancer cells. The osteolytic lesion area was significantly larger in CtpsK/BSP mice than in the controls as demonstrated by both radiographic and histomorphometric analyses. TRAP staining demonstrated a twofold increase in the number of osteoclasts in the bone lesion area from CtpsK/BSP mice compared with that from wild type mice. We conclude that host tissue-derived BSP also plays important roles in breast cancer metastasis through inducing tumor cell seeding into the remote host tissues. Furthermore, osteoclast-derived BSP promotes osteoclast differentiation in an autocrine manner and consequently promotes osteolytic bone metastasis of breast cancer.

Haploinsufficiency of Runx2 results in bone formation decrease and different BSP expression pattern changes in two transgenic mouse models

Runx2 has been identified as "a master gene" for the differentiation of osteoblasts and Runx2-deficient mice has demonstrated a complete absence of mature osteoblast and ossification. To further characterize the Runx2 responsive elements within the bone sialoprotein (BSP) promoter and further investigate into the role of Runx2 haploinsufficiency in osteoblast differentiation, mBSP9.0Luc mice and mBSP4.8Luc mice were crossed with Runx2-deficient mice respectively. Luciferase assay, micro CT scan, and histological analysis were performed using tissues isolated from mBSP9.0luc/Runx2+/- mice, mBSP4.8luc/Runx2+/- mice and their corresponding Runx2+/+ littermates. Alkaline phosphatase activity, mineralization assays and RT-PCR analysis using calvarial osteoblasts isolated from these transgenic mice were also performed. Luciferase assay demonstrated an early increase in luciferase expression in mBSP9.0luc/Runx2+/- mice before the expression level of luciferase dramatically decreased and turned lower than that in their control littermates in later stages. In contrast, luciferase expression in mBSP4.8luc/Runx2+/- failed to show such an early increase. Micro CT scan and histological analysis showed that BMD and trabecular bone volume were decreased and bone formation was delayed in Runx2+/- mice. Furthermore, mineralization assay and semi-quantitative RT-PCR assay demonstrated a gene-dose-dependent decrease in bone nodule formation and bone marker genes expression levels in cultured calvarial osteoblasts derived from Runx2 knockout mice. Reconstitution of Runx2-null cells with Runx2 vector partially rescued the osteoblast function defects. In conclusion, the 9.0 kb BSP promoter demonstrated a higher tissue-specific regulation of the BSP gene by Runx2 in vivo and full Runx2 gene dose is essential for osteoblast differentiation and normal bone formation.

Bone sialoprotein and its transcriptional regulatory mechanism

BACKGROUND AND OBJECTIVE

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

MATERIAL AND METHODS

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

RESULTS

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

CONCLUSION

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

Systemically transplanted bone marrow stromal cells contributing to bone tissue regeneration

Bone marrow stromal cells (BMSCs) are a rich source of osteogenic progenitor cells. A fundamental question is whether systemically transplanted BMSCs participate in bone regeneration. Luciferase and GFP double-labeled BMSCs were transplanted into irradiated mice. Five weeks after transplantation, artificial bone wounds were created in the mandibles and calvaria of the recipients. Animals were sacrificed at weeks 2, 4, and 6 after surgery and the expressions of luciferase and GFP were determined using Xenogen IVIS Imaging System, immunohistochemical staining and RT-PCR. The results demonstrated that transplanted BMSCs can be detected in wound sites as early as 2 weeks and lasted the whole experimental period. Luciferase expression peaked at 2 weeks after surgery and decreased thereafter, exhibiting a similar expression pattern as that of BSP, while GFP expression was relatively stable during the experimental period. In conclusion, BMSCs can migrate to bone wound sites and participate in bone regeneration in orocraniofacial region.

Conditionally replicating adenovirus-mediated gene therapy in bladder cancer: an orthotopic in vivo model

INTRODUCTION

The effects of a conditionally replicating adenovirus on various bladder cancer lines were explored, a truncated bone sialoprotein (BSP) promoter controlling the E1a/b lytic-regulating sequence was used, since BSP protein is found in many osteotropic neoplasms, including bladder cancer.

METHODS

Reverse transcriptase polymerase chain reaction analysis was used to determine expression patterns of BSP and Coxsackie adenovirus receptor, a receptor known to interact with adenovirus, on multiple lines of bladder cancer (253J, 253J B-V, RT4, transitional cell carcinoma, T24, UMUC3, and WH). Ad-BSP-E1a was tested in vitro for lytic activity on 4 of these cell lines. The 253J B-V cell line was used and inoculated into female nude mice either subcutaneously in the flank or orthotopically into the bladder, and treated with control or Ad-BSP-E1a virus.

RESULTS

BSP is expressed in RT4, transitional cell carcinoma, and WH. Meanwhile, Coxsackie adenovirus receptor was expressed in all lines except T24. Ad-BSP-E1a had the most impact on 253J and 253J B-V cells; cell density declined significantly when compared to phosphate-buffered saline and Ad-BSP-TK "dummy" virus-treatment groups. The 253J B-V tumors treated with Ad-BSP-E1a revealed a decreased percent change of size in the subcutaneous model when compared to controls at week 3. The orthotopic murine model showed decreased end tumor mass in the Ad-BSP-E1a treated group over controls. Histologic examination of in vivo tumors showed evidence of fibrosis and apoptosis in the Ad-BSP-E1a treated groups using hematoxylin-eosin, trichrome, and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling (TUNEL) staining. Control groups only had viable tumor in in vivo models.

CONCLUSION

Adenovirus therapy of orthotopic murine bladder tumors is feasible. Ad-BSP-E1a is effective in treating very aggressive yet sensitive bladder tumor cells. Further study of this conditionally replicating adenovirus treatment (Ad-BSP-E1a) with chemotherapeutic combination is warranted, and future translation of such combination therapy into human beings is a possibility.

Porous silk fibroin 3-D scaffolds for delivery of bone morphogenetic protein-2 in vitro and in vivo

Bone morphogenetic protein-2 (BMP-2) plays a key role in osteogenesis. Biomaterials used for the sustained delivery of BMP-2 in vivo have shown therapeutic benefits. In the present study, BMP-2 was loaded in porous silk fibroin scaffolds derived from silkworm cocoons (2.4 +/- 0.14 microg per scaffold). The release profile of BMP-2 under dynamic culture conditions (spinner flasks) showed that after 1 week in culture 25% of the initial BMP-2 was retained adsorbed to the scaffold; up to 4 weeks no additional BMP-2 was released. BMP-2 induced human bone marrow stromal cells (hMSCs) to undergo osteogenic differentiation when the seeded scaffolds were cultured in medium supplemented with osteogenic stimulants for 4 weeks, based on elevated alkaline phosphatase activity, calcium deposition, and transcript levels for bone sialoprotein, osteopontin, osteocalcin, BMP-2, and cbfa-1. Micro-computed tomography revealed densely deposited mineral at the center of the scaffolds. In contrast, hMSCs cultured in control scaffolds (no BMP-2) exhibited limited osteogenesis. When implanted in critical sized cranial defects in mice, scaffolds loaded with BMP-2 and seeded with hMSCs resulted in significant bone ingrowth. These results were qualitatively similar to scaffolds loaded with BMP-2 but no hMSCs or with BMP-2 and hMSCs but not pregrown into bone-like tissue. Bone-related outcomes were improved when compared with the scaffold controls implanted without BMP-2. These studies illustrate the potential use of slow degrading silk fibroin 3-D scaffolds loaded with BMP-2, in combination with hMSCs, in osteogenesis studies in vitro and in vivo, and provide a new range of material properties for these applications.

An in vivo model to study osteogenic gene regulation: targeting an avian retroviral receptor (TVA) to bone with the bone sialoprotein (BSP) promoter

To study bone development in vivo, a transgenic mouse model was established in which an avian retroviral receptor (TVA) gene driven by the BSP promoter was selectively expressed in skeletal tissues. The model was validated by showing suppressed BSP expression and delayed bone and tooth formation after infection with a virus expressing a mutated Cbfa1/Runx2 gene.

INTRODUCTION

Tissue-specific expression of the avian retroviral (TVA) receptor can be used to efficiently target ectopic expression of genes in vivo. To determine the use of this approach for studies of osteogenic differentiation and bone formation at specific developmental stages, transgenic mice expressing the TVA receptor under the control of a 5-kb bone sialoprotein (BSP) promoter were generated. The mice were first analyzed for tissue-specific expression of the TVA gene and then, after infection with a viral construct, for the effects of a dominant-negative form of the Cbfa1/Runx2 transcription factor on bone formation.

MATERIALS AND METHODS

We first generated transgenic mice (BSP/TVA) in which the TVA gene was expressed under the control of a 4.9-kb mouse BSP promoter. The tissue-specific expression of the TVA gene was analyzed by RT-PCR, in situ hybridization, and immunohistochemistry and compared with the expression of the endogenous BSP gene. A 396-bp fragment of mutated Cbfa1/Runx2 (Cbfa1mu) encoding the DNA-binding domain was cloned into a RCASBP (A) viral vector, which was used to infect neonatal BSP/TVA mice.

RESULTS AND CONCLUSION

Expression of the TVA receptor mRNA and protein in the transgenic mice was consistent with the expression of endogenous BSP. Four days after systemic infection with the Cbfa1mu-RCASBP (A) vector, RT-PCR analyses revealed that the expression of BSP mRNA in tibia and mandibles was virtually abolished, whereas a 30% reduction was seen in calvarial bone. After 9 days, BSP expression in the tibia and mandible was reduced by 45% in comparison with control animals infected with an empty RCASBP vector, whereas BSP expression in the membranous bone of calvariae was decreased approximately 15%. However, after 4 and 8 weeks, there was almost no change in BSP expression in any of the bone tissues. In comparison, a reduction in osteopontin expression was only observed 9 days after viral transfection in the three bones. Histomorphological examination revealed that bone formation and tooth development were delayed in some of the mice infected with mutated Cbfa1. These studies show that BSP/TVA transgenic mice can be used to target genes to sites of osteogenesis, providing a unique system for studying molecular events associated with bone formation in vivo.

Cooperative interactions between RUNX2 and homeodomain protein-binding sites are critical for the osteoblast-specific expression of the bone sialoprotein gene

The bone sialoprotein (Bsp) gene provides an excellent model for studying mechanisms controlling osteoblast-specific gene expression. Although the RUNX2 transcription factor directly regulates many osteoblast-related genes, its function in Bsp expression remains uncertain. By using chromatin immunoprecipitation (ChIP) analysis in MC3T3-E1 (clone MC-4) preosteoblast cells, RUNX2 was shown to bind a chromatin fragment containing the proximal Bsp promoter. Two putative RUNX2-binding sites (R1 and R2) were identified within this region of the mouse, rat, and human genes and were shown to bind RUNX2 in vitro and in vivo (by ChIP assay). Site-specific mutagenesis established that both sites act as osteoblast-specific transcriptional enhancers and together account for nearly two-thirds of the total promoter activity. In addition, functional cooperativity was observed between the R2 site and an adjacent homeodomain protein-binding site previously characterized by this laboratory (the C site). All three sites (R1, R2, and C) are necessary for maximal promoter activity in osteoblasts. DLX5 in MC-4 cell nuclear extracts binds to the C site in vitro. Furthermore, ChIP assays revealed that DLX5 is selectively associated with chromatin in the vicinity of the C site only when Bsp is transcriptionally active. Finally, co-immunoprecipitation assays detected a physical complex containing DLX5 and RUNX2. Taken together, our data show that RUNX2 is a direct regulator of Bsp in osteoblasts and that it functions in cooperation with DLX5 or a related factor to activate osteoblast-specific gene expression.

Identification of promoter regions involved in cell- and developmental stage-specific osteopontin expression in bone, kidney, placenta, and mammary gland: an analysis of transgenic mice

Cell-specific expression of GFP under the control of different lengths of the osteopontin promoter in transgenic mice identified the positive and negative regulatory regions for respective cell types. The results provide new insights for physiological and pathological expression of the osteopontin gene.

INTRODUCTION

Osteopontin (OPN) is a major non-collagenous bone matrix protein that is involved in normal and pathological calcification and is expressed in a tissue-specific manner. To investigate how such tissue-specific OPN gene expression is regulated in vivo, transgenic mice expressing the green fluorescent protein (GFP) reporter gene controlled by different lengths of the OPN promoter were generated.

MATERIALS AND METHODS

Cell- and developmental stage-specific osteopontin expression in transgenic mice was examined by Northern blotting, immunoblotting, fluorescence examination, and in situ hybridization and compared with those of OPN.

RESULTS AND CONCLUSIONS

The line bearing the -5505 to +14 region of the OPN promoter was shown by Northern blotting and immunoblotting to express GFP in the same cells that express endogenous OPN (osteoblasts, hypertrophic chondrocytes, renal and mammary gland epithelial cells, and granulated metrial gland [GMG] placental cells) at the same stage in development. Thus, the 5.5-kb -5505 to +14 promoter region is sufficient for proper tissue-specific OPN expression. The lines carrying shorter segments of the OPN promoter showed different expression patterns. These patterns revealed a putative cis-acting element in the -5269 to -5263 region that restricts OPN expression to hypertrophic chondrocytes and a mammary gland-specific expressing element and a GMG cell-specific enhancing element in the -5505 to -3156 region. Furthermore, the -3155 to -1576 region seems to contain positive renal epithelial cell- and GMG cell-specific expression motif(s) as well as a negative regulatory element that prevents OPN expression in fibroblasts. Moreover, the -1576 to -910 region seems to contain a positive osteoblast-specific-expressing element. Thus, the 5.5-kb OPN promoter contains multiple cis-acting elements encoding positive and negative cell-specific regulatory systems.

An L1 element disrupts human bone sialoprotein promoter: lack of tissue-specific regulation by distalless5 (Dlx5) and runt homeodomain protein2 (Runx2)/core binding factor a1 (Cbfa1) elements

Bone sialoprotein (BSP) is a phosphorylated and sulphated glycoprotein with hydroxyapatite nucleating properties that is specifically expressed in association with physiological and pathological mineralization. Although previous studies have indicated that tissue-specific expression of murine BSP is regulated through a proximal homeodomain element that binds distalless5 (Dlx5) transcription analysis of the homologous human promoter revealed modest enhancement in osteogenic cells. Moreover, whereas forced expression of an antisense Dlx5 vector increased transcription, Dlx5 expression did not alter transcription significantly. Since extended promoter sequences are required to confer absolute tissue-specific expression of BSP in vivo, we characterized the upstream region of the human BSP gene. In contrast to the rat and mouse promoters, which show conserved sequences extending several kbs upstream, analysis of approximately 3 kb of the human promoter showed no sequence conservation beyond -0.99 kb. Southern blot analysis of genomic DNA from four different BAC clones showed that this sequence was not an aberration in the human genomic library used to isolate the BSP gene. Using clone BAC H-NH0811I08, the human BSP promoter sequence was extended approximately 8 kb upstream from which the non-homologous region was characterized as a 3.48 kb insert coding for an L1 retrotransposon element. Transcriptional analyses of chimeric promoter constructs revealed that the retrotransposon element suppresses transcription <80%. Upstream of the inserted DNA several regions, varying in length from 26 to 161 bps, were conserved within the mouse and human promoters. One of these conserved regions included a runt homeodomain protein2 (Runx2)/core binding factor a1 (Cbfa1) elements consensus element in reverse orientation. Whereas a multimeric form of the element was transcriptionally active in response to Runx2/Cbfa1 when ligated to the BSP basal promoter, the single element in the context of the extended promoter was unresponsive. These studies have characterized the upstream promoter of the human BSP gene, which is interrupted by a unique high-frequency DNA insert that suppresses BSP gene transcription.

Real-time visualization of MMP-13 promoter activity in transgenic mice

Cutaneous wound repair involves extracellular matrix degradation, cell migration, matrix resynthesis and tissue remodeling. In the rodent, transcriptional regulation of collagenase-3 (MMP-13) most likely plays a role in these processes. Therefore, we isolated and characterized a 1.76-kb 5'-flanking region of the mouse MMP-13 gene. Assay of promoter activity by transient transfection of HT1080 cells and primary mouse skin fibroblasts allowed identification of several functional regions of the 5'-flanking DNA. Expression of luciferase reporter constructs in these cells was induced by phorbol myristate acetate (PMA), but not by transforming growth factor-beta(2) (TGF-beta(2)). To study the regulation of MMP-13 in cutaneous wound healing, we generated transgenic mouse lines harboring the firefly luciferase reporter gene under control of a 660-bp mouse MMP-13 promoter which showed maximal response. MMP-13 mRNA levels in transgenic lung fibroblasts increased 1.5-2.6-fold after PMA challenge. MMP-13 promoter activity in wounds was visualized and quantified in vivo as luciferase bioluminescence. MMP-13 expression was present at day 1 and maximal at day 18 post-wounding. Luciferase activity progressed from the wound margin towards the center of the wound. In situ hybridization showed the same spatial and temporal patterns for the luciferase and endogenous MMP-13 mRNA. Both signals localized predominantly to dermal fibroblasts at the wound periphery but not to granulation tissue or to keratinocytes. These results suggested that MMP-13 participated in the wound healing of acute wounds, and it was a significant factor in long-term remodeling of wound connective tissue in rodent skin.

Col1a1-driven transgenic markers of osteoblast lineage progression

The modular organization of the type I collagen promoter allows creation of promoter-reporter constructs with preferential activity in different type I collagen-producing tissues that might be useful to mark cells at different stages of osteoblastic differentiation. Primary marrow stromal cell (MSC) and mouse calvarial osteoblast (mCOB) cultures were established from transgenic mice harboring different Col1a1 promoter fragments driving chloramphenicol acetyltransferase (CAT). In these models, Col1a1 messenger RNA (mRNA) and alkaline phosphatase (ALP) are the first markers of differentiation appearing soon after the colonies develop. Bone sialoprotein (BSP) is detected 2-3 days later, followed by osteocalcin (OC) expression and nodule mineralization. A 3.6 Col1a1 fragment (ColCAT3.6) initiated activity concomitant with ALP staining and type I collagen mRNA expression. In contrast, a 2.3 Col1a1 fragment (ColCAT2.3) became active coincident with BSP expression. The pattern of transgene expression assessed by immunostaining was distinctly different. ColCAT3.6 was expressed within and at the periphery of developing nodules whereas the ColCAT2.3 expression was restricted to the differentiated nodules. The feasibility of using green fluorescent protein (GFP) as a marker of osteoblast differentiation was evaluated in ROS17/2.8 cells. A 2.3-kilobase (kb) Col1a1 promoter driving GFP (pOB4Col2.3GLP) was stably transfected into the cell line and positive clones were selected. Subcultures lost and then regained GFP expression that was localized in small clusters of cells throughout the culture. This suggests that expression from the 2.3-kb Col1A1 fragment is determined by the state of differentiation of the ROS17/2.8 cells. Col1a1 transgenes should be useful in appreciating the heterogeneity of a primary or immortalized culture undergoing osteoblastic differentiation.

Expression of bone sialoprotein in mineralized tissues of tooth and bone and in buccal-pouch carcinomas of Syrian golden hamsters

The expression of bone sialoprotein (BSP) is normally restricted to mineralized connective tissues of bones and teeth where it has been associated with mineral crystal formation. However, recent studies have revealed ectopic expression of BSP in various lesions, including oral and extraoral carcinomas, in which it has been associated with the formation of microcrystalline deposits and the metastasis of cancer cells to bone. To develop a model to study the induction of BSP in carcinoma development, BSP expression in squamous-cell carcinomas induced by chemical carcinogen in the hamster cheek-pouch epithelium was investigated. Hamster BSP cDNA was first isolated and characterized, then used to prepare probes for Northern and in situ hybridization. The protein sequence of hamster BSP displayed 86% amino acid identity with a consensus mammalian BSP sequence and retained polyglutamate sequences, the RGD sequence and sites of phosphorylation, glycosylation and sulphation. The tissue-specific expression of hamster BSP mRNA and protein was confirmed by in situ hybridization and immunolocalization in developing tissues. Squamous-cell carcinomas induced in the buccal pouches of 5-week-old male Syrian golden hamsters treated with chemical carcinogen had BSP mRNA and BSP in the proliferating neoplastic epithelium. In contrast, neither BSP mRNA nor the protein could be detected in the stroma within which islands of the transformed tissue had formed. Thus, the hamster cheek pouch is a well-characterized model that can be used to study the induced expression of BSP in association with the development of squamous-cell carcinomas.

Identification of a homeodomain binding element in the bone sialoprotein gene promoter that is required for its osteoblast-selective expression

Bone sialoprotein is a 70-kDa extracellular matrix component that is intimately associated with biomineralization, yet the cis-acting elements of the Bsp gene that restrict its expression to mineralizing cells remain uncharacterized. To identify such elements, we analyzed a 2472-base pair fragment of the murine promoter that directs osteoblast-selective expression of a luciferase reporter gene and found that the region between -338 and -178 relative to the transcriptional start is crucial for its osteoblast-selective activity. We identified an element within this region that binds a protein complex in the nuclear extracts of osteoblastic cells and is required for its transcriptional activity. Introduction of a mutation that disrupts a homeodomain binding site within this sequence eliminates both its in vitro binding and nearly all of the osteoblastic-selective activity of the 2472-base pair promoter. We further found that the Dlx5 homeoprotein, which is able to regulate the osteoblast-specific osteocalcin promoter, can bind this element and stimulate its enhancer activity when overexpressed in COS7 cells. These data represent the first description of an osteoblast-specific element within the bone sialoprotein promoter and demonstrate its regulation by a member of a family of factors known to be involved in skeletogenesis.

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.

Characterization of Osf1, an osteoblast-specific transcription factor binding to a critical cis-acting element in the mouse Osteocalcin promoters

To elucidate the mechanisms of osteoblast-specific gene expression we are studying the regulation of osteocalcin, the most osteoblast-specific gene. Previous studies of OG2, one of the two mouse osteocalcin genes, identified two osteoblast-specific cis-acting elements, OSE1 and OSE2, the latter being the binding site of Cbfa1, the only osteoblast-specific transcription factor known to date. Here we show that OSE1 is a cis-acting element as important as OSE2 for the osteoblast-specific expression of OG2 in cell culture and transgenic mice. We also show that OSE1 is present in the promoter of several osteoblast-specific genes including Cbfa1 itself. These biological features demonstrate the importance of OSE1 and led us to further characterize this site and the factor binding to it, provisionally termed Osf1. We first defined the core OSE1 sequence, 5'-TTACATCA-3', which is necessary and sufficient for Osf1 binding to DNA. This sequence has no strong homology to any known transcription factor-binding sites. As a first step in identifying Osf1, we performed an analytical purification of this protein using nuclear extracts from two different osteoblastic cell lines. We purified Osf1 to homogeneity through a five-step procedure including a renaturation experiment and found that its apparent molecular mass is 40 kDa. In conclusion, this study indicates the existence of multiple osteoblast-specific cis-acting elements of equal importance in controlling OG2 promoter activity and provides the first biochemical characterization of Osf1, a novel osteoblast-specific transcription factor.

Osteomimetic properties of prostate cancer cells: a hypothesis supporting the predilection of prostate cancer metastasis and growth in the bone environment

BACKGROUND

Unlike most other malignancies, prostate cancer metastasizes preferentially to the skeleton and elicits osteoblastic reactions.

METHODS

We present a hypothesis, based upon results obtained from our laboratory and others, on the nature of progression of prostate cancer cells and their predilection to growth and metastasis in the bone microenvironment. We propose the hypothesis that osseous metastatic prostate cancer cells must be osteomimetic in order to metastasize, grow, and survive in the skeleton. The reciprocal interaction between prostate cancer and bone stromal growth factors, including basic fibroblast growth factor (bFGF), hepatocyte growth factor/scatter factor (HGF/SF), and especially the insulin growth factor (IGF) axis initiates bone tropism, and is enhanced by prostate secreted endothelin-1 (ET-1) and urokinase-type plasminogen activator (uPA). Growth factors and peptides that have differentiating activity, such as transforming growth factor beta (TGF-beta), parathyroid hormone-related protein (PTH-rp), and the bone morphogenetic proteins (BMPs), can shift local homeostasis to produce the characteristic blastic phenotype, via interaction with prostate-secreted human kalikrein 2 (hK2), and prostate-specific antigen (PSA). This proposal asserts that altering the expression of certain critical transcription factors, such as Cbfa and MSX in prostate cancer cells, which presumably are under the inductive influences of prostate or bone stromal cells, can confer profiles of gene expression, such as osteopontin (OPN), osteocalcin (OC), and bone sialoprotein (BSP), that mimic that of osteoblasts.

RESULTS AND CONCLUSIONS

Elucidation of common proteins, presumably driven by the same promoters, expressed by both prostate cancer and bone stromal cells, could result in the development of novel preventive and therapeutic strategies for the treatment of prostate cancer skeletal metastasis. Agents developed using these strategies could have the potential advantage of interfering with growth and enhancing apoptosis in both prostate cancer and bone stromal compartments. The selective application of gene therapy strategy, driven by tissue-specific and tumor-restricted promoters for the safe delivery and expression of therapeutic genes in experimental models of prostate cancer metastasis, is discussed.

Cloning of a 2.5 kb murine bone sialoprotein promoter fragment and functional analysis of putative Osf2 binding sites

Bone sialoprotein (BSP) is an extracellular matrix protein that is intimately associated with the process of biomineralization. Osf2, a member of the Cbf/runt family of transcription factors, is required for the development of osteoblasts in vivo and has been reported to stimulate the transcription of BSP when overexpressed in mesenchymal cell lines. To investigate the role of Osf2 in BSP expression, we cloned a 2.5 kb fragment of a 5' untranscribed sequence from the murine BSP gene and evaluated it for putative Osf2 binding sites. This promoter, which was able to direct 5- to 10-fold higher levels of luciferase reporter expression in osteoblastic cells than in nonbone cell lines, contains two consensus core binding sites for members of the Cbf/runt family. One, at -61 relative to the start of transcription, is within a region having 75% overall sequence identity with the rat and human BSP promoters. The other is located at -1335, outside this highly conserved region. Neither site is completely conserved in the rat or human sequences. Only the -1335 site was able to bind a protein in nuclear extracts of osteoblastic cells, and this protein was identified as Osf2. Despite this in vitro binding ability, we detected no significant enhancer activity in the -1335 element when placed in front of a minimal osteocalcin promoter driving a luciferase reporter gene in osteoblastic cells nor any loss in transcriptional activity of a 5' promoter deletion which eliminated this element as compared with the full-length 2.5 kb promoter. These results suggest that Osf2 binding to the BSP promoter is not essential for its osteoblast-selective expression.

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

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

The effect of altered functional forces on the expression of bone-matrix proteins in developing mouse mandibular condyle

Mechanical forces are known to have an effect on bone formation, maintenance and remodelling, and there is evidence that the development of the mandibular condyle in the rat is influenced by the consistency of the diet. Here a mouse model was used to investigate the relation between food, condylar development and the expression of bone sialoprotein (BSP), osteopontin (OPN), osteocalcin (OC) and type 1 collagen (COL I). Twenty-four 19-day-old male mice were randomly divided into three groups. Groups 1 and 2 were fed hard pellets and soft powdered food, respectively, for 2 weeks. Group 3 mice were fed soft food for 1 week followed by a week of hard pellets. Incisors of mice in groups 2 and 3 were trimmed twice a week to reduce occlusal forces. After killing the animals, mandibular condyles were collected for RNA extraction, in situ hybridization and immunohistochemical analyses. Histological sections showed that the condyles of mice in group 2 were underdeveloped, with a thinner layer of cartilage and fewer bone trabeculae. Northern hybridization of total RNA of the condyle from mice in this soft-food group also exhibited a significant decrease in the amounts of BSP, OPN, OC and COL I, representing 79%, 75%, 77% and 79% respectively, of that from mice fed hard food. In situ hybridization of these bone-matrix proteins demonstrated signals in bone-forming cells and BSP mRNA was also seen in the hypertrophic cartilage cells in the developing condyle. Immunohistochemical study demonstrated an obvious difference in the intensity of staining, especially for BSP. Results from group 3 were similar to those from group 1. The observed decrease in bone matrix-protein expression confirms that the consistency of the diet affects the development of the mouse mandibular condyle and that a soft diet diminishes the rate of bone formation.

Tissue specific and vitamin D responsive gene expression in bone

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

Neoplastic odontogenic epithelial cells express bone sialoprotein

Bone sialoprotein (BSP) is synthesized and secreted by bone-, dentine- and cementum-forming cells and has been implicated in de novo bone formation and mineralization. In this study, we used histological sections of odontogenic neoplasms and performed immunohistochemical and in situ hybridization analyses. In ameloblastoma, BSP mRNA signals were seen in the neoplastic epithelial cells forming nests, strips and islands. BSP deposition was also seen in the stellate reticulum of the tumour masses revealed by immunohistochemistry using human BSP antibodies. In calcifying epithelial odontogenic tumour, the calcified masses demonstrated positive immunoreactivity to the human BSP antibodies, and the hybridization signals for BSP were located in the cells near the calcified particles. In the calcifying odontogenic cyst, strong BSP signals were seen in cells surrounding the characteristic nests of ghost cells, which often calcify subsequently. BSP protein was also found in these cells by immunohistochemistry. The active expression of BSP in the epithelial elements of the odontogenic tumours of adult patients suggests the activation of this matrix protein gene in the neoplastic process, and that BSP may play an important role in tumour formation and differentiation with respect to pathological calcification.

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

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