The murine gene encoding secreted phosphoprotein 1 (osteopontin): promoter structure, activity, and induction in vivo by estrogen and progesterone

Regulation and function of osteopontin in ras-transformed cells

Transformation of nontumorigenic NIH 3T3 fibroblasts with an activated ras oncogene produces malignant cells that can metastasize. This induction of malignant behavior is due to changes in gene expression induced by Ras-mediated signal transduction. Osteopontin expression is induced in response to Ras, due to increased osteopontin transcription mediated by Ras-responsive regions in the osteopontin gene promoter. The increased expression of osteopontin contributes functionally to the malignant ability of the cells. Ras-transformed cells that express antisense osteopontin RNA show markedly reduced ability to form tumors and to metastasize in experimental animals. Increased osteopontin expression in human tumors thus may also contribute to increased malignancy. Site-directed mutagenesis of recombinant osteopontin protein indicates that an intact RGD sequence is required for cell adhesion and induction of chemotaxis, consistent with the idea that integrin-mediated signal transduction is a consequence of osteopontin binding to cells. Osteopontin may contribute to malignancy by inducing responses in host and/or tumor cells.

Osteopontin-induced modifications of cellular functions

Osteopontin (OPN) serves both a cell attachment function and a cell signalling function via the alpha v beta 3 integrin. We have investigated the action on mammalian cells of recombinant OPN made both in E. coli and in human cells. In its cell signalling capacity it initiates a signal transduction cascade that includes changes in the intracellular calcium ion levels and the tyrosine phosphorylation status of several proteins including pp60src and components of focal adhesion complexes. Effects on gene expression include suppression of the induction of nitric oxide synthase by inflammatory mediators. OPN can also reduce cell peroxide levels, promote the survival of cells exposed to hypoxia, and inhibit the killing of tumor cells by activated macrophages.

Structure of the osteopontin gene and its promoter

We cloned the hOPN gene and its 5' upstream region, and analyzed its exon-intron structure and potential regulatory sequences of the promoter region in comparison with those of mouse and porcine homologues. The hOPN gene consists of 7 exons that are similar to those of the mouse gene, although the hOPN gene is longer than the mouse homologue. This difference is attributable to an insertion of about 1750 bp immediately before exon 4 in the hOPN gene. A region of approximately 285 bp immediately upstream of the hOPN transcription initiation site was highly conserved and contained a number of potential cis regulatory consensus sequences. CAT analysis using SCC-3 cells demonstrated that nucleotides at positions -439 to -270, -124 to -80, and -55 to -39 contained cis-acting enhancing elements, in which the -124 to -80 element was much more active than the others. Deletion of the sequences between -474 and -270 localized the cis elements to the sequence at position -439 to -410, whereas the deletion between -124 to -80 localized it to -124 to -115, and -94 to -80 (data not shown). Gel shift analysis using synthesized double-stranded oligonucleotides corresponding to the 30 bp at position -439 to -410 (data not shown), and 10 and 15 bp regions at positions -124 to -115 and -94 to -80, respectively, as probes revealed that each probe formed one or two bands complexed with a nuclear protein prepared from SCC-3 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a ras-activated enhancer in the mouse osteopontin promoter and its interaction with a putative ETS-related transcription factor whose activity correlates with the metastatic potential of the cell

The role of RAS in transducing signals from an activated receptor into altered gene expression is becoming clear, though some links in the chain are still missing. Cells possessing activated RAS express higher levels of osteopontin (OPN), an alpha v beta 3 integrin-binding secreted phosphoprotein implicated in a number of developmental, physiological, and pathological processes. We report that in T24 H-ras-transformed NIH 3T3 cells enhanced transcription contributes to the increased expression of OPN. Transient transfection studies, DNA-protein binding assays, and methylation protection experiments have identified a novel ras-activated enhancer, distinct from known ras response elements, that appears responsible for part of the increase in OPN transcription in cells with an activated RAS. In electrophoretic mobility shift assays, the protein-binding motif GGAGGCAGG was found to be essential for the formation of several complexes, one of which (complex A) was generated at elevated levels by cell lines that are metastatic. Southwestern blotting and UV light cross-linking studies indicated the presence of several proteins able to interact with this sequence. The proteins that form these complexes have molecular masses estimated at approximately 16, 28, 32, 45, 80, and 100 kDa. Because the approximately 16-kDa protein was responsible for complex A formation, we have designated it MATF for metastasis-associated transcription factor. The GGANNNAGG motif is also found in some other promoters, suggesting that they may be similarly controlled by MATF.

Cloning and characterization of the human osteopontin gene and its promoter

We isolated the human osteopontin (hOP) gene and the 5' upstream region, and analysed its exon-intron structure and potential regulatory sequences of the promoter region in comparison with those of the mouse and porcine gene. The coding sequence is split into 7 exons which are similar to those of the mouse gene, although the hOP gene is longer than the mouse gene. The difference in length is mainly due to variations in intron 3, which is approximately 2.7-fold longer than that of the mouse OP gene. The 5' upstream region of the hOP, which is highly conserved up to nucleotide -250, contains a number of potential cis regulatory consensus sequences. A series of sequentially 5'-deleted chimeric clones was tested for the ability to stimulate chloramphenicol acetyltransferase (CAT). Initial CAT analysis demonstrated that nucleotides at positions -474 to -270, -124 to -80, and -55 to -39 contained cis-acting enhancing sequences in a human monocyte cell line, SCC-3, although the -124 to -80 region was much more active than other regions. Deletion of the sequences between -474 and -270 localized this cis region to the sequence at positions -439 to -410, whereas the deletion between -124 to -80 localized the regions to -124 to -115, and -94 to -80. Gel-shift analysis using as probes synthesized double-stranded DNA corresponding to the 10 and 15 bp region at positions -124 to -115 and -94 to -80 respectively revealed that each probe formed a major band complexed with nuclear proteins prepared from SCC-3 cells.

Characterization of the chicken osteopontin-encoding gene

A genomic clone of the chicken osteopontin-encoding gene (opn) was isolated and found to be organized as follows: an untranslated 5' exon; a signal peptide; a recognition sequence for phosphorylation by casein kinase II; a domain containing a possible O-linkage site for glycosylation; a second casein kinase II phosphorylation site; an exon containing three functional regions, the poly-Asp sequence of seven consecutive Asp residues, the RGD integrin recognition site and a potential N-linkage site for glycosylation; and a large C-terminal exon which also contains a potential N-linkage site for glycosylation. Primer extension analysis demonstrated only one strong transcriptional start point (tsp) in mRNAs prepared from embryonic bone and cultured osteoblasts. Analysis of the 5' flanking region identified a TATA sequence at -31, an inverted CAAT motif at -57, an AP1-recognition sequence at -84 and a putative vitamin-D-response element (VDRE) sequence at -474. Three plasmid constructs containing 963, 561 and 368 bp of 5' flanking sequence of the avian promoter were used to drive expression of bacterial cat. Comparison of the relative promoter activities of these constructs was carried out in MC3T3/E1 cells, a murine osteoblast cell line. All of the constructs showed approximately 20-fold the levels of expression over background activity of the cat gene without a promoter. Each construct also demonstrated a strong induction with phorbol-12-myristyl-13-acetate (PMA). In contrast, dihydroxycholecalciferol [1,25(OH)2D3] had neither a positive nor a negative effect on the 368- and 936-bp constructs, but was stimulatory for the 561-bp construct.(ABSTRACT TRUNCATED AT 250 WORDS)

Box I and II motif from myelin basic protein gene promoter binds to nuclear proteins from rodent brain

The box I and II motif located within the promoter of the myelin basic protein gene contains a simian virus 40 T-antigen-binding site, a MyoD/E2a-binding site, and a glucocorticoid receptor-binding site. We have found proteins within nuclear extracts from adult mouse brain, rat embryonic cerebral cortex in culture, and a mouse oligodendrocyte-like cell line that bind to a 32P-labeled synthetic DNA fragment containing the sequences of the box I and II motif. Three major complexes (A, B, and C) were seen in gel-shift assays. Only complexes A and B were competed out by the unlabeled box I and II fragment or by another synthetic DNA fragment that also contains sequences similar to a glucocorticoid receptor-binding site. Therefore, complexes A and B were thought to be specific. The expression pattern of the proteins responsible for the formation of these complexes was also assessed during development in mouse brain.

Characterization of the stromal osteogenic cell line MN7: mRNA steady-state level of selected osteogenic markers depends on cell density and is influenced by 17 beta-estradiol

The steady-state mRNA levels of different osteogenic markers and their modulation by 17 beta-estradiol in the murine osteogenic cell line MN7 during proliferation and differentiation in vitro were examined. mRNA of collagen type I, osteopontin, bone morphogenetic protein 2, plasminogen activator inhibitor 1, alkaline phosphatase, and osteocalcin were isolated from MN7 cultures grown for 7, 11, 14, and 17 days. Northern blot analysis revealed steady-state transcript levels depending on MN7 cell density. The order of appearance of Col I, OP, ALP, and OC resembled the pattern of gene expression observed during osteoblast maturation in vitro. Furthermore, PAI-1 steady-state transcript levels peaked during subconfluence (day 11) but BMP-2 RNA levels reached their maximum after the culture had become confluent. 17 beta-Estradiol showed a dose-dependent stimulation of the different osteoblast-related transcripts present in a subconfluent MN7 culture at the time of analysis. Furthermore, the effects of 17 beta-estradiol (17 beta E2) at different time points of MN7 growth varied according to cell density. 17 beta E2 added to subconfluent MN7 cultures modulated the transcript level in a negative way, but RNA levels of the investigated osteogenic markers in confluent cultures were stimulated with 100 nM 17 beta-estradiol. No effect of 17 beta-estradiol on proliferation was detected. The present studies have revealed differential osteoblast gene expression related to MN7 cell proliferation and differentiation in vitro and emphasize the importance of 17 beta E2 in the regulation of growth of this preosteoblastic cell line in vitro.

Osteoblasts are target cells for transformation in c-fos transgenic mice

We have generated transgenic mice expressing the proto-oncogene c-fos from an H-2Kb class I MHC promoter as a tool to identify and isolate cell populations which are sensitive to altered levels of Fos protein. All homozygous H2-c-fosLTR mice develop osteosarcomas with a short latency period. This phenotype is specific for c-fos as transgenic mice expressing the fos- and jun-related genes, fosB and c-jun, from the same regulatory elements do not develop any pathology despite high expression in bone tissues. The c-fos transgene is not expressed during embryogenesis but is expressed after birth in bone tissues before the onset of tumor formation, specifically in putative preosteoblasts, bone-forming osteoblasts, osteocytes, as well as in osteoblastic cells present within the tumors. Primary and clonal cell lines established from c-fos-induced tumors expressed high levels of exogenous c-fos as well as the bone cell marker genes, type I collagen, alkaline phosphatase, and osteopontin/2ar. In contrast, osteocalcin/BGP expression was either low or absent. All cell lines were tumorigenic in vivo, some of which gave rise to osteosarcomas, expressing exogenous c-fos mRNA, and Fos protein in osteoblastic cells. Detailed analysis of one osteogenic cell line, P1, and several P1-derived clonal cell lines indicated that bone-forming osteoblastic cells were transformed by Fos. The regulation of osteocalcin/BGP and alkaline phosphatase gene expression by 1,25-dihydroxyvitamin D3 was abrogated in P1-derived clonal cells, whereas glucocorticoid responsiveness was unaltered. These results suggest that high levels of Fos perturb the normal growth control of osteoblastic cells and exert specific effects on the expression of the osteoblast phenotype.

Purification and characterization of three proteins isolated from the proteose peptone fraction of bovine milk

Three major proteins from the proteose peptone of bovine milk were purified by Sephadex G-75 gel chromatography, Q-Sepharose ion-exchange and additional Sephadex G-75 gel chromatography in the presence of urea. From their mobility in a gradient SDS-PAGE, the proteins were found to have molecular masses of 17, 28 and 60 kDa. The N-terminal amino acid sequence of the 17 kDa protein was found to be homologous with a camel whey protein. This protein has not previously been described in bovine milk. From the SDS-PAGE results, the 28 kDa protein was judged to be the major protein of proteose peptone, contributing approximately 25% of the total. The N-terminal amino acid sequence showed no homology to any known protein sequence, but the amino acid composition indicated that the 28 kDa protein is identical with the PP3 component from the proteose peptone fraction of bovine milk, or part of it. The 60 kDa protein was found to be bovine osteopontin, a very highly phosphorylated protein with an Arg-Gly-Asp sequence which mediates cell attachment.

p21ras and protein kinase C function in distinct and interdependent signaling pathways in C3H 10T1/2 fibroblasts

Both p21ras and protein kinase C (PKC) are believed to function downstream of plasma membrane-associated tyrosine kinases in cellular signal transduction pathways. However, it has remained controversial whether they function in the same pathway and, if so, what their relative position and functional relationship in such a pathway are. We investigated the possibilities that p21ras and PKC function either upstream or downstream of each other in a common linear pathway or that they function independently in colinear signal pathways. Either decreased expression of endogenous normal ras in fibroblasts transfected with an inducible antisense ras construct or overexpression of a mutant ras gene reduced the capacity of the phorbol ester tetradecanoyl phorbol acetate to trigger expression of the tetradecanoyl phorbol acetate-responsive and ras-dependent reporter gene osteopontin (OPN). PKC depletion decreased basal OPN mRNA levels, and the overexpression of ras restored OPN expression to the level of non-PKC-depleted cells. We propose a model in which ras and PKC function in distinct and interdependent signaling pathways.

Developmental expression of osteopontin (OPN) mRNA in rat tissues: evidence for a role for OPN in bone formation and resorption

Osteopontin (OPN) is a 34-kDa, highly-phosphorylated glycoprotein with cell attachment properties that is a prominent constituent of the bone matrix. To aid in elucidating the function of this protein we have studied the cellular expression of OPN mRNA during the formation, growth and maturation of rat calvarial (membranous) and tibial (endochondral) bone. From Northern hybridization analysis OPN expression was demonstrated in the kidney and gravid uterus as well as in bone tissues. Compared to collagen, the expression of OPN was low in early bone formation but increased subsequently and reached peak levels in 14-day-old bone. However, both the collagen and OPN mRNAs decreased markedly thereafter and remained low in young adult bone. From in situ hybridization studies using a [35S]-labelled rat OPN cRNA probe, OPN mRNA was localized to osteoblastic cells in newly-forming calvariae, jaw bones, and in the metaphyseal and periosteal bone of the tibia. In contrast to bone sialoprotein (BSP), which is expressed almost exclusively by osteoblasts at sites of de novo bone formation, OPN transcripts were present in cells lining both endosteal and periosteal bone surfaces, and in osteocytes. Moreover, expression of OPN persisted during the subsequent growth and remodelling of both membranous and endochondral bone and was expressed at particularly high levels by bone cells and hypertrophic chondrocytes at sites of osteoclastic resorption. In the more mature bone of young adult rats OPN expression was significantly reduced but remained detectable in bone cells lining periosteal and endosteal surfaces and in the primary and secondary spongiosa of the tibia. These studies on the developmental expression of OPN support the concept of a multifunctional role for OPN in bone formation and remodelling. Thus, the expression of OPN by osteoblasts early in bone development is consistent with a role for this protein in the formation of bone matrix, whereas the peak expression of OPN later in bone development, together with high expression at sites of rapid remodelling, indicate that OPN deposited on the surface of mineralized connective tissues may provide a template for osteoclastic resorption.

p21ras and protein kinase C function in distinct and interdependent signaling pathways in C3H 10T1/2 fibroblasts

Both p21ras and protein kinase C (PKC) are believed to function downstream of plasma membrane-associated tyrosine kinases in cellular signal transduction pathways. However, it has remained controversial whether they function in the same pathway and, if so, what their relative position and functional relationship in such a pathway are. We investigated the possibilities that p21ras and PKC function either upstream or downstream of each other in a common linear pathway or that they function independently in colinear signal pathways. Either decreased expression of endogenous normal ras in fibroblasts transfected with an inducible antisense ras construct or overexpression of a mutant ras gene reduced the capacity of the phorbol ester tetradecanoyl phorbol acetate to trigger expression of the tetradecanoyl phorbol acetate-responsive and ras-dependent reporter gene osteopontin (OPN). PKC depletion decreased basal OPN mRNA levels, and the overexpression of ras restored OPN expression to the level of non-PKC-depleted cells. We propose a model in which ras and PKC function in distinct and interdependent signaling pathways.

Tumor progression and metastasis in murine D2 hyperplastic alveolar nodule mammary tumor cell lines

We have examined tumor progression and metastatic properties of three clonal murine mammary tumor cell lines of recent origin (D2A1, D2.OR and D2.1). These lines were derived from spontaneous mammary tumors which originated from a D2 hyperplastic alveolar nodule (HAN) line. D2A1 cells were more malignant than D2.OR or D2.1 cells, whether measured by experimental metastasis assays after intravenous injection in nude mice or chick embryos, in vivo growth rate of primary tumors following mammary fat pad injection in nude mice, or spontaneous metastasis assay from primary tumors growing in mammary fat pads. D2A1 cells also were more invasive in vitro in a Matrigel invasion assay than D2.1 cells, while the D2.OR cells were non-invasive in this assay. The increased invasiveness and malignancy of D2A1 cells were associated with increased levels of mRNA for the cysteine proteinase cathepsin L. Levels of osteopontin (OPN), nm23, int-1 and int-2 mRNAs were also examined. Nm23 levels were highest in the most malignant cell line. These cell lines provide a model for studying the tumorigenic and metastatic ability of mammary tumor cells and offer several advantages: they were cloned from mammary tumors that originate from a common source of preneoplastic cells (D2HAN); they are of relatively recent origin; and they have spontaneously arrived at different stages of tumor progression.

Expression and distribution of osteopontin in human tissues: widespread association with luminal epithelial surfaces

Osteopontin, a glycoprotein with a glycine-arginine-glycine-aspartate-serine (GRGDS) cell-binding domain, has been described in bone and is also known to be expressed in other organs, particularly kidney. The goal of the present work was to define the distribution of osteopontin synthesis and deposition in a wide variety of normal adult human tissues using a multifaceted approach that included immunohistochemistry, in situ hybridization, and Northern analysis. Immunohistochemical studies have revealed the unexpected finding that osteopontin is deposited as a prominent layer at the luminal surfaces of specific populations of epithelial cells of the gastrointestinal tract, gall bladder, pancreas, urinary and reproductive tracts, lung, breast, salivary glands, and sweat glands. Northern analyses identified gallbladder as a major site of osteopontin gene transcription comparable in magnitude with that of kidney, and immunoblotting identified osteopontin in bile. In situ hybridization localized osteopontin gene transcripts predominantly to the epithelium of a variety of organs as well as to ganglion cells of bowel wall. Osteopontin of epithelial cell origin, like bone-derived osteopontin, promoted GRGDS-dependent cell spreading in attachment assays. We postulate that osteopontin secreted by epithelium binds integrins on luminal surfaces. Collectively, these findings suggest an important role for osteopontin on many luminal epithelial surfaces communicating with the external environment.

Regulated temporal and spatial expression of the calcium-binding proteins calcyclin and OPN (osteopontin) in mouse tissues during pregnancy

In situ hybridization and northern/slot blot analyses were used to quantify the expression of calcyclin (2A9, 5B10), osteopontin (opn, secreted phosphoprotein, 2ar) and calmodulin mRNAs in mouse tissues that support pregnancy. High-to-moderate levels of the mRNAs of all three genes were detected at discrete locations in the uterus, decidua and placenta as a function of gestation time. Calmodulin expression was constant in these tissues; calcyclin mRNA was high during early pregnancy and declined after day 8-9 of gestation; and opn mRNA was undetectable before day 7, with maximal levels on days 9-12 in each of these tissues. Calcyclin, but not opn, expression was also observed in the chorioamnion after day 12. Calcyclin was expressed throughout the decidua on day 8 but became restricted to the primary (antimesometrial) decidual zone and decidua lateralis on day 9, and the decidua capsularis after day 9. By contrast, opn mRNA was localized on day 9 to the mesometrial triangle, which contains a large population of granulated metrial gland cells, and to the decidua basalis. These two genes may serve as markers for the two types of decidual tissue. We suggest that one function of OPN, which may be an indicator of cells in the decidua that have a bone marrow genealogy, is to mediate the flux of calcium from the maternal circulation to the developing embryo.

Characterization of the promoter region of the porcine opn (osteopontin, secreted phosphoprotein 1) gene. Identification of positive and negative regulatory elements and a 'silent' second promoter

Osteopontin (secreted phosphoprotein-1, Opn) is a phosphorylated glycoprotein expressed by transformed cells, macrophages, activated T-lymphocytes, specialized epithelial cells and bone cells that is characteristically enriched in milk and in the mineralized matrix of bone. The synthesis of Opn by bone cells is regulated by glucocorticoids and growth factors, which promote bone formation, and by the osteotropic hormone calcitriol (1,25-dihydroxycholecalciferol) and retinoic acid, which mediate bone resorption, indicating a bifunctional role for this protein in bone remodelling. To study the transcriptional regulation of the opn gene, two genomic clones (10 and 15 kb) encoding the opn gene were isolated from a porcine liver genomic library cloned into lambda phage. From the 15-kb clone a 4-kb EcoRI fragment containing the first two exons and 2.6 kb of the 5' flanking region of the opn gene was sequenced, and the transcriptional start site determined by primer extension analysis and S1 nuclease mapping. To identify the opn promoter, chimeric chloramphenicol acetyltransferase constructs were prepared using fragments from the first intron and the 5' flanking region of the opn gene. Transient transfection of porcine bone cells with these constructs showed strong promoter activity located within 74 bp upstream from the transcription initiation site. Within this region a TATA sequence, TTTAAA, was identified at positions -26 to -31. However, the highest transcription rate was observed in a construct extending 180 bp upstream that included a CCGCCC Sp1 binding sequence (-63 to -68), and an AP1 site (-74 to -80). Further upstream in the 5' flanking region and within the first intron of the opn, a number of consensus sequences could be identified. Chimeric constructs containing a GGGTCAtatGGTTCA direct repeat consensus sequence for a vitamin D3 response element located at nucleotides -2245 to -2259 responded to the addition of 0.1 microM calcitriol by a 2.5-fold stimulation of transcription, although a greater than 2-fold increase was also observed in shorter constructs -180 to -905 lacking such a consensus sequence. Promoter activity was also exhibited by a region containing a TTTAAA sequence in the first intron that corresponded to the putative promoter site reported for mouse opn in macrophages (Miyazaki, Y., Setoguchi, M., Yoshida, S., Higuchi, Y., Akizuki, S. & Yamamoto, S. (1990) J. Biol. Chem. 265, 14432-14438).(ABSTRACT TRUNCATED AT 400 WORDS)

The cDNA cloning and RNA distribution of bovine osteopontin

We have isolated and sequenced the bovine cDNA (OPN) counterpart of osteopontin. The cDNA is 1356 nucleotides (nt) in length with an open reading frame of 834 nt, encoding a 278-amino acid (aa) protein. Cell-free transcription and translation of OPN RNA resulted in a major species of approx. 40 kDa in size, in agreement with the predicted size of the deduced aa sequence. Northern analysis of bovine OPN RNA indicated the presence of the message in mineralized, as well as soft tissues. A comparison of the deduced aa sequence among various species indicates both regions of similarity and divergence. One prominent region of dissimilarity in bovine OPN compared to all other species is a 22-aa gap which may represent a loss of a potential Ca(2+)-binding loop. Despite the variability among the species, several regions of conservation are apparent, including a hydrophobic leader sequence, a potential site for Asn-linked glycosylation, a stretch of polyaspartic acid residues, and the cell attachment Arg-Gly-Asp tripeptide. Whether bovine OPN enhances cell attachment is unknown. Furthermore, whether the loss of a potential Ca(2+)-binding loop alters the function of OPN would be interesting to determine.