Identification of a novel response element in the rat bone sialoprotein (BSP) gene promoter that mediates constitutive and fibroblast growth factor 2-induced expression of BSP

Fibroblast growth factor signaling controlling osteoblast differentiation

Fibroblast growth factors (FGFs) play important roles in skeletal development and postnatal osteogenesis. FGF signaling controls bone formation by regulating the expression of various genes involved in osteoprogenitor cell replication, osteoblast differentiation and apoptosis. Recent genetic manipulation of FGF expression in mice and studies of the phenotype induced by gain-of-function mutations in FGF receptors in humans revealed the important role of FGF signaling in osteoblast function and differentiation. Additionally, cell biology studies allowed to identify some signaling pathways that are involved in the control of FGF actions in osteoblasts. This led to a better understanding of the functional role of FGF signaling in the control of gene expression in osteoblasts. The elucidation of molecular mechanisms by which FGF signaling promotes osteoblast gene expression and differentiation may help to find novel molecular targets and develop new therapeutic approaches to promote bone formation in human bone disorders.

Prostaglandin E2 stimulates bone sialoprotein (BSP) expression through cAMP and fibroblast growth factor 2 response elements in the proximal promoter of the rat BSP gene

Bone sialoprotein (BSP), an early marker of osteoblast differentiation, has been implicated in the nucleation of hydroxyapatite during de novo bone formation. Prostaglandin E2 (PGE2) has anabolic effects on proliferation and differentiation of osteoblasts via diverse signal transduction systems. Because PGE2 increases the proportion of functional osteoblasts in fetal rat calvarial cell cultures, we investigated the regulation of BSP, as an osteoblastic marker, by PGE2. Treatment of rat osteosarcoma UMR 106 cells with 3 microm, 300 nm, and 30 nm PGE2 increased the steady state levels of BSP mRNA about 2.7-, 2.5-, and 2.4-fold after 12 h. From transient transfection assays, the constructs including the promoter sequence of nucleotides (nt) -116 to +60 (pLUC3) were found to enhance transcriptional activity 3.8- and 2.2-fold treated with 3 microm and 30 nm PGE2 for 12 h. 2-bp mutations were made in an inverted CCAAT box (between nt -50 and -46), a cAMP response element (CRE; between nt -75 and -68), a fibroblast growth factor 2 response element (FRE; nt -92 to -85), and a pituitary-specific transcription factor-1 motif (between nt -111 and -105) within pLUC3 and pLUC7 constructs. Transcriptional stimulation by PGE2 was almost completed abrogated in constructs that included 2-bp mutations in either the CRE and FRE. In gel shift analyses an increased binding of nuclear extract components to double-stranded oligonucleotide probes containing CRE and FRE was observed following treatment with PGE2. These studies show that PGE2 induces BSP transcription in UMR 106 cells through juxtaposed CRE and FRE elements in the proximal promoter of the BSP gene.

Regulation of the osteoblast-specific transcription factor, Runx2: responsiveness to multiple signal transduction pathways

The Cbfa1/Runx2 is an important transcription factor necessary for osteoblast differentiation and bone formation. However, the signaling pathways regulating Runx2 activity are just beginning to be understood. Inconsistencies between Runx2 mRNA or protein levels and its transcriptional activity suggests that posttranslational modification and/or protein-protein interactions may regulate this factor. Runx2 can be phosphorylated and activated by the mitogen-activated protein kinase (MAPK) pathway. This pathway can be stimulated by a variety of signals including those initiated by extracellular matrix (ECM), osteogenic growth factors like bone morphogenic proteins (BMPs) and fibroblast growth factor-2 (FGF-2), mechanical loading and hormones such as parathyroid hormone (PTH). Protein kinase A (PKA) may also phosphorylate/activate Runx2 under certain conditions. In addition, Runx2 activity is enhanced by protein-protein interactions as are seen with PTH-induced Runx2/AP-1 and BMP-mediated Runx2/Smads interactions. Mechanisms for interaction with Runx2 are complex including binding of distinct components such as AP-1 factors and Smads proteins to separate DNA regions in target gene promoters and direct physical interactions between Runx2 and AP-1/Smad factors. Post-translational modifications such as phosphorylation may influence interactions between Runx2 and other nuclear factors. These findings suggest that Runx2 plays a central role in coordinating multiple signals involved in osteoblast differentiation.

Activation of bone sialoprotein gene transcription by flavonoids is mediated through an inverted CCAAT box in ROS 17/2.8 cells

Bone sialoprotein (BSP) is a major noncollagenous protein of the mineralized bone extracellular matrix that has been implicated in the nucleation of hydroxyapatite. Recent studies have shown that BSP is also expressed by osteotropic cancers suggesting BSP might play a role in the pathogenesis of bone metastases. The present study investigates regulation of BSP transcription in rat osteosarcoma ROS 17/2.8 cells by flavonoids: genistein (an inhibitor of protein tyrosine kinases), daidzein (an inactive compound of genistein), flavone, and flavanone. Genistein, daidzein, and flavone (50 microM) increased steady state levels of BSP mRNA about 1.7-fold at 12 h. From transient transfection assays using various sized BSP promoter-luciferase constructs, genistein increased luciferase activities within 12 h. Constructs including the promoter sequence nucleotides (nts) -116 to -43 (pLUC3) were found to enhance transcriptional activity approximately 2.6-fold in ROS 17/2.8 cells treated with genistein (50 microM). Daidzein, flavone, and flavanone (50 microM) also increased luciferase activities. In contrast, the tyrosine kinase inhibitors, herbimycin A and lavendustin A, which do not have a flavonoid structure, did not stimulate BSP transcription. Transcriptional stimulation by genistein was almost completely abrogated in a construct that included 2 bp mutations in the inverted CCAAT box. A monoclonal antibody against NF-YA, a CCAAT box-binding transcription factor, inhibited formation of DNA-NF-Y protein complex in gel shift assays formed by nuclear extracts of ROS 17/2.8 cells. These data suggest that the inverted CCAAT box is required for flavonoid-induced BSP expression and that the stimulatory action is dependent on the flavone structure and does not involve an inhibitory action on protein tyrosine kinase.

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.

Fibroblast growth factor 2 induction of the osteocalcin gene requires MAPK activity and phosphorylation of the osteoblast transcription factor, Cbfa1/Runx2

Fibroblast growth factor 2 (FGF-2) is an important regulator of bone formation and osteoblast activity. However, its mechanism of action on bone cells is largely unknown. A major route for FGF signaling is through the mitogen-activated protein kinase (MAPK) pathway. We showed recently that this pathway is important for activation and phosphorylation of Cbfa1/Runx2, an osteoblast-related transcription factor (Xiao, G., Jiang, D., Thomas, P., Benson, M. D., Guan, K., Karsenty, G., and Franceschi, R. T. (2000) J. Biol. Chem. 275, 4453-4459). The present study examined the mechanism of FGF-2 regulation of the mouse osteocalcin gene in MC3T3-E1 preosteoblastic cells. FGF-2 stimulated osteocalcin mRNA and promoter activity in a dose- and time-dependent manner in MC3T3-E1 preosteoblastic cells. Similar results were obtained in mouse bone marrow stromal cells. This stimulation required Runx2 and its DNA binding site in the osteocalcin promoter. FGF-2 also dramatically increased phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) followed by phosphorylation of Runx2. Furthermore, a specific ERK1/2 phosphorylation inhibitor, U0126, completely blocked both FGF-2-stimulated Runx2 phosphorylation and osteocalcin promoter activity, indicating that this regulation requires the MAPK pathway. Deletion studies showed that the C-terminal PST domain of Runx2 is required for the FGF-2 response. This study is the first demonstration that Runx2 is phosphorylated and activated by FGF-2 via the MAPK pathway and suggests that FGF-2 plays an important role in regulation of Runx2 function and bone formation.

Tumor necrosis factor alpha (TNF-alpha)-induced prostaglandin E2 release is mediated by the activation of cyclooxygenase-2 (COX-2) transcription via NFkappaB in human gingival fibroblasts

Nuclear factor kappaB (NFkappaB) is a transcription factor and plays a key role in the expression of several genes involved in the inflammatory process. Cyclooxygenase (COX) is the key regulatory enzyme of the prostaglandin/eicosanoid synthetic pathway. COX-2 is a highly inducible enzyme by proinflammatory cytokines, of which gene expression is regulated by NFkappaB. TNF-alpha is a pro-inflammatory cytokine. In this paper, we investigated the involvement of NFkappaB on TNF-alpha-mediated prostaglandin E2 (PGE2) release and COX-2 gene expression in human gingival fibroblasts (HGF). TNF-alpha-induced PGE2 release and COX-2 mRNA accumulation in a time- and concentration-dependent manner in HGF. The results of transient transfection assays using a chimeric construct of the human COX-2 promoter (nts -1432 approximately +59) ligated to a luciferase reporter gene indicated that TNF-a stimulated the transcriptional activity approximately 1.4-fold. Gel mobility shift assays with a radiolabelled COX-2-NFkappaB oligonucleotide (nts -223 to -214) revealed an increase in the binding of nuclear proteins from TNF-alpha-stimulated HGF. The COX-2-NFKB DNA-protein complex disappeared after treatment with pyrrolidine dithiocarbamate (PDTC; an antioxidant) or herbimycin A (a tyrosine kinase inhibitor). PDTC and herbimycin A attenuated TNF-alpha-stimulated PGE2 release. These results suggest that NFkappaB transcription factor is a key regulator of COX-2 expression in TNF-alpha-induced PGE2 production, which is mediated through a tyrosine kinase pathway in HGF.