Expression of the osteonectin gene potentially controlled by multiple cis- and trans-acting factors in cultured bone cells

A single nucleotide polymorphism in osteonectin 3' untranslated region regulates bone volume and is targeted by miR-433

Osteonectin/SPARC is one of the most abundant noncollagenous extracellular matrix proteins in bone, regulating collagen fiber assembly and promoting osteoblast differentiation. Osteonectin-null and haploinsufficient mice have low-turnover osteopenia, indicating that osteonectin contributes to normal bone formation. In male idiopathic osteoporosis patients, osteonectin 3' untranslated region (UTR) single-nucleotide polymorphism (SNP) haplotypes that differed only at SNP1599 (rs1054204) were previously associated with bone mass. Haplotype A (containing SNP1599G) was more frequent in severely affected patients, whereas haplotype B (containing SNP1599C) was more frequent in less affected patients and healthy controls. We hypothesized that SNP1599 contributes to variability in bone mass by modulating osteonectin levels. Osteonectin 3' UTR reporter constructs demonstrated that haplotype A has a repressive effect on gene expression compared with B. We found that SNP1599G contributed to an miR-433 binding site, and miR-433 inhibitor relieved repression of the haplotype A, but not B, 3' UTR reporter construct. We tested our hypothesis in vivo, using a knock-in approach to replace the mouse osteonectin 3' UTR with human haplotype A or B 3' UTR. Compared with haplotype A mice, bone osteonectin levels were higher in haplotype B mice. B mice displayed higher bone formation rate and gained more trabecular bone with age. When parathyroid hormone was administered intermittently, haplotype B mice gained more cortical bone area than A mice. Cultured marrow stromal cells from B mice deposited more mineralized matrix and had higher osteocalcin mRNA compared with A mice, demonstrating a cell-autonomous effect on differentiation. Altogether, SNP1599 differentially regulates osteonectin expression and contributes to variability in bone mass, by a mechanism that may involve differential targeting by miR-433. This work validates the findings of the previous candidate gene study, and it assigns a physiological function to a common osteonectin allele, providing support for its role in the complex trait of skeletal phenotype. © 2014 American Society for Bone and Mineral Research.

A new reporter vector system based on flow-cytometry to detect promoter activity

In this study, we report the development of a new dual reporter vector system for the analysis of promoter activity. This system employs green fluorescence emitting protein, EGFP, as a reporter, and uses red fluorescence emitting protein, DsRed, as a transfection control in a single vector. The expression of those two proteins can be readily detected via flow cytometry in a single analysis, with no need for any further manipulation after transfection. As this system allows for the simultaneous detection of both the control and reporter proteins in the same cells, only transfected cells which express the control protein, DsRed, can be subjected to promoter activity analysis, via the gating out of all un-transfected cells. This results in a dramatic increase in the promoter activity detection sensitivity. This novel reporter vector system should prove to be a simple and efficient method for the analysis of promoter activity.

Thrombospondin-2 and SPARC/osteonectin are critical regulators of bone remodeling

Thrombospondin-2 (TSP2) and osteonectin/BM-40/SPARC are matricellular proteins that are highly expressed by bone cells. Mice deficient in either of these proteins show phenotypic alterations in the skeleton, and these phenotypes are most pronounced under conditions of altered bone remodeling. For example, TSP2-null mice have higher cortical bone volume and are resistant to bone loss associated with ovariectomy, whereas SPARC-null mice have decreased trabecular bone volume and fail to demonstrate an increase in bone mineral density in response to a bone-anabolic parathyroid hormone treatment regimen. In vitro, marrow stromal cell (MSC) osteoprogenitors from TSP2-null mice have increased proliferation but delayed formation of mineralized matrix. Similarly, in cultures of SPARC-null MSCs, osteoblastic differentiation and mineralized matrix formation are decreased. Overall, both TSP2 and SPARC positively influence osteoblastic differentiation. Intriguingly, both of these matricellular proteins appear to impact MSC fate through mechanisms that could involve the Notch signaling system. This review provides an overview of the role of TSP2 and SPARC in regulating bone structure, function, and remodeling, as determined by both in vitro and in vivo studies.

miR-29 suppression of osteonectin in osteoblasts: regulation during differentiation and by canonical Wnt signaling

The matricellular protein osteonectin, secreted protein acidic and rich in cysteine (SPARC, BM-40), is the most abundant non-collagenous matrix protein in bone. Matricellular proteins play a fundamental role in the skeleton as regulators of bone remodeling. In the skeleton, osteonectin is essential for the maintenance of bone mass and for balancing bone formation and resorption in response to parathyroid hormone (PTH). It promotes osteoblast differentiation and cell survival. Mechanisms regulating the expression of osteonectin in the skeleton and in other tissues remain poorly understood. We found that the proximal region of the mouse osteonectin 3' untranslated region (UTR) contains a well-conserved, dominant regulatory motif that interacts with microRNAs (miRs)-29a and -29c. Transfection of osteoblastic cells with miR-29a inhibitors increased osteonectin protein levels, whereas transfection of miR-29a precursor RNA decreased osteonectin. miR-29a and -29c were increased during osteoblastic differentiation in vitro. The up-regulation of these miRNAs correlated with decreased osteonectin protein during the matrix maturation and mineralization phases of late differentiation. In contrast, osteonectin transcript levels remained relatively constant during this process, implying repression of translation. Treatment of osteoblasts with LiCl induced miR-29a and -29c expression and decreased osteonectin synthesis. When cells were treated with Dickkopf-1 (Dkk-1), miR-29a and -29c expression was repressed. These data suggest that canonical Wnt signaling, which is increased during osteoblastic differentiation, induces expression of miR-29. Osteonectin and miR-29 are co-expressed in extra-skeletal tissues, and the post-transcriptional mechanisms regulating osteonectin in osteoblasts are likely to be active in other cell systems.

Osteonectin/SPARC polymorphisms in Caucasian men with idiopathic osteoporosis

Animal models suggest a role for osteonectin/SPARC in determination of bone mass. We found haplotypes consisting of three single nucleotide polymorphisms (SNPs) in the 3' untranslated region (UTR) of the osteonectin gene are associated with bone density in Caucasian men with idiopathic osteoporosis.

INTRODUCTION

Osteonectin is a matricellular protein regulating matrix assembly, osteoblast differentiation, and survival. Animal studies indicate that osteonectin is essential for normal bone mass. The 3' UTR is a regulatory region controlling mRNA stability, trafficking, and translation, and we determined whether osteonectin 3' UTR haplotypes could be associated with bone mass and/or idiopathic osteoporosis.

METHODS

Single strand conformation polymorphism and allele-specific PCR analysis were used to assess alleles at osteonectin cDNA bases 1046, 1599, and 1970, using genomic DNA from middle-aged Caucasian men with idiopathic, low turnover osteoporosis (n = 56) and matched controls (n = 59). Bone density was measured by DXA at spine, hip and radius. Allele and haplotype frequencies were analyzed by Chi square analysis and Fisher's exact test.

RESULTS

Five common osteonectin 3' UTR haplotypes were identified. The frequency of one haplotype (1046C-1599C-1970T) was higher in controls compared with patients, and this haplotype was also associated with higher bone densities at multiple sites in patients. In contrast, a second haplotype (1046C-1599G-1970T) was associated with lower bone densities in patients at multiple sites.

CONCLUSIONS

Osteonectin regulates skeletal remodeling and bone mass in animals, and haplotypes in the 3' UTR of this gene are associated with bone density in Caucasian men with idiopathic osteoporosis.

Tumor associated stromal cells play a critical role on the outcome of the oncolytic efficacy of conditionally replicative adenoviruses

The clinical efficacy of conditionally replicative oncolytic adenoviruses (CRAd) is still limited by the inefficient infection of the tumor mass. Since tumor growth is essentially the result of a continuous cross-talk between malignant and tumor-associated stromal cells, targeting both cell compartments may profoundly influence viral efficacy. Therefore, we developed SPARC promoter-based CRAds since the SPARC gene is expressed both in malignant cells and in tumor-associated stromal cells. These CRAds, expressing or not the Herpes Simplex thymidine kinase gene (Ad-F512 and Ad(I)-F512-TK, respectively) exerted a lytic effect on a panel of human melanoma cells expressing SPARC; but they were completely attenuated in normal cells of different origins, including fresh melanocytes, regardless of whether cells expressed or not SPARC. Interestingly, both CRAds displayed cytotoxic activity on SPARC positive-transformed human microendothelial HMEC-1 cells and WI-38 fetal fibroblasts. Both CRAds were therapeutically effective on SPARC positive-human melanoma tumors growing in nude mice but exhibited restricted efficacy in the presence of co-administered HMEC-1 or WI-38 cells. Conversely, co-administration of HMEC-1 cells enhanced the oncolytic efficacy of Ad(I)-F512-TK on SPARC-negative MIA PaCa-2 pancreatic cancer cells in vivo. Moreover, conditioned media produced by stromal cells pre-infected with the CRAds enhanced the in vitro viral oncolytic activity on pancreatic cancer cells, but not on melanoma cells. The whole data indicate that stromal cells might play an important role on the outcome of the oncolytic efficacy of conditionally replicative adenoviruses.

Gap junctional communication modulates gene expression in osteoblastic cells

Bone-forming cells are organized in a multicellular network interconnected by gap junctions. In these cells, gap junctions are formed by connexin43 (Cx43) and connexin45 (Cx45). Cx43 gap junctions form pores that are more permeable to negatively charged dyes such as Lucifer yellow and calcein than are Cx45 pores. We studied whether altering gap junctional communication by manipulating the relative expression of Cx43 and Cx45 affects the osteoblast phenotype. Transfection of Cx45 in cells that express primarily Cx43 (ROS 17/2.8 and MC3T3-E1) decreased both dye transfer and expression of osteocalcin (OC) and bone sialoprotein (BSP), genes pivotal to bone matrix formation and calcification. Conversely, transfection of Cx43 into cells that express predominantly Cx45 (UMR 106-01) increased both cell coupling and expression of OC and BSP. Transient cotransfection of promoter-luciferase constructs and connexin expression vectors demonstrated that OC and BSP gene transcription was down-regulated by Cx45 cotransfection in ROS 17/2. 8 and MC3T3-E1 cells, in association with a decrease in dye coupling. Conversely, cotransfection of Cx43 in UMR 106-01 cells up-regulated OC and BSP gene transcription. Activity of other less specific osteoblast promoters, such as osteopontin and osteonectin, was less sensitive to changes in gap junctional communication. Thus, altering gap junctional permeability by manipulating the expression of Cx43 and Cx45 in osteoblastic cells alters transcriptional activity of osteoblast-specific promoters, presumably via modulation of signals that can diffuse from cell to cell. A communicating intercellular network is required for the full elaboration of a differentiated osteoblastic phenotype.

Identification of an intronic enhancer that nullifies upstream repression of SPARC gene expression

The SPARC gene 5'flanking sequence has been shown to contain enhancer elements, but also negative control elements immediately upstream of the enhancer elements. Although these 5'enhancer elements are active in F9 and PYS-2 cells, their activities are nullified by the 5'repressor activity. In the present study we have identified within intron 1 between nucleotides (nt) +5000 and +5150 of the SPARC gene an enhancer element that bound to two transcription factors of 48 and 52 kDa and between nt +5000 and +5523 a DNase I hypersensitive site. Furthermore, a region containing the 3'intron 1 enhancer element, together with the 5'enhancer elements, neutralized the 5'repressor activity and stimulated efficient transcription. The resulting SPARC promoter activity is about equal in F9, differentiated F9 and PYS-2 cells. We consistently found that the rate of SPARC transcription is nearly the same in F9 and PYS-2 cells. Association of the 3'enhancer element in intron 1 with the DNase I hypersensitive site suggests that both play a role in regulating SPARC expression in vivo .

A purine-rich sequence in the human BM-40 gene promoter region is a prerequisite for maximum transcription

BM-40 (osteonectin, SPARC [secreted protein, acidic, rich in cysteine]) is a highly conserved, matrix-associated protein that is found in basement membranes, bones and remodeling tissues throughout vertebrate evolution. We are reporting the characterization of the 5' end of the human BM-40 gene. Sequence comparison of the 5' region revealed significant homologies with the bovine and murine genes, including a purine-rich stretch composed of two boxes, GGA-box 1 and 2, separated by a pyrimidine-rich spacer element. Transfection analyses of the human BM-40 promoter provide strong evidence that this region comprises several distinct regulatory domains, to which different functions can be assigned. GGA-box 1 is thereby absolutely required and sufficient by itself for maximal BM-40 transcriptional activity, whereas the spacer element has a down-regulatory effect. Comparative transfection analyses in human cell lines, positive or negative for BM-40 transcripts, indicate that the GGA-box sequences in the human promoter, in contrast to the bovine promoter, do not significantly contribute to cell-type specific expression in human cells.

Partial characterization of a novel 'GGA' factor which binds to the osteonectin promoter in bovine bone cells

Osteonectin (On)/SPARC (secreted protein, acidic and rich in cysteine) is a highly conserved extracellular matrix protein found in bone and other tissues throughout vertebrate evolution. In previous studies, approximately 500 bp of DNA 5' to the transcription start point (tsp) and a part of exon 1, including homopurine (Rn)/homopyrimidine (Yn)-rich sequences (the 'GGA box' and its complements), was demonstrated to be important in upregulation of On gene expression in a cell-specific manner. The purpose of this study was to decipher the transcriptional regulation of On through its cis- and trans-acting elements. DNase I footprinting analysis indicated protein binding which may be related to the transcriptional factors, AP2, SP1 and a novel 'GGA' factor which binds to the 3' end of the promoter (-286 to +43 to the tsp). Comparisons of footprinting between nuclear extracts of bone (On-expressing) cells and Madin and Darby bovine kidney (MDBK) (nonexpressing) cells indicate that 'GGA' factor binding to a purine GGGGA/GGA-rich sequence is cell-type specific and therefore may be involved in the cell-specific expression of this gene. From ultraviolet (UV)-crosslinking experiments, this 'GGA' factor was demonstrated to be a single 40-kDa protein.