Identification and characterization of two proximal elements in the rat osteocalcin gene promoter that may confer species-specific regulation

Bad to the Bone: The Effects of Therapeutic Glucocorticoids on Osteoblasts and Osteocytes

Despite the continued development of specialized immunosuppressive therapies in the form of monoclonal antibodies, glucocorticoids remain a mainstay in the treatment of rheumatological and auto-inflammatory disorders. Therapeutic glucocorticoids are unmatched in the breadth of their immunosuppressive properties and deliver their anti-inflammatory effects at unparalleled speed. However, long-term exposure to therapeutic doses of glucocorticoids decreases bone mass and increases the risk of fractures - particularly in the spine - thus limiting their clinical use. Due to the abundant expression of glucocorticoid receptors across all skeletal cell populations and their respective progenitors, therapeutic glucocorticoids affect skeletal quality through a plethora of cellular targets and molecular mechanisms. However, recent evidence from rodent studies, supported by clinical data, highlights the considerable role of cells of the osteoblast lineage in the pathogenesis of glucocorticoid-induced osteoporosis: it is now appreciated that cells of the osteoblast lineage are key targets of therapeutic glucocorticoids and have an outsized role in mediating their undesirable skeletal effects. As part of this article, we review the molecular mechanisms underpinning the detrimental effects of supraphysiological levels of glucocorticoids on cells of the osteoblast lineage including osteocytes and highlight the clinical implications of recent discoveries in the field.

Glucocorticoid-induced osteoporosis in children with 21-hydroxylase deficiency

21-Hydroxylase deficiency (21-OHD) is the most common cause of congenital adrenal hyperplasia (CAH), resulting from deletions or mutations of the P450 21-hydroxylase gene (CYP21A2). Children with 21-OHD need chronic glucocorticoid (cGC) therapy, both to replace congenital deficit in cortisol synthesis and to reduce androgen secretion by adrenal cortex. GC-induced osteoporosis (GIO) is the most common form of secondary osteoporosis that results in an early, transient increase in bone resorption accompanied by a decrease in bone formation, maintained for the duration of GC therapy. Despite the conflicting results in the literature about the bone status on GC-treated patients with 21-OHD, many reports consider these subjects to be at risk for osteoporosis and fractures. In bone cells, at the molecular level, GCs regulate various functions including osteoblastogenesis, osteoclastogenesis, and the apoptosis of osteoblasts and osteocytes. In this paper, we focus on the physiology and biosynthesis of endogenous steroid hormones as well as on the effects of GCs on bone cells, highlighting the pathogenetic mechanism of GIO in children with 21-OHD.

The transcriptional activity of osterix requires the recruitment of Sp1 to the osteocalcin proximal promoter

The transcription factor osterix (Osx/Sp7) is required for osteogenic differentiation and bone formation in vivo. While Osx can act at canonical Sp1 DNA-binding sites and/or interact with NFATc1 to cooperatively regulate transcription in some osteoblast promoters, little is known about the molecular details by which Osx regulates osteocalcin (OCN) transcription. We previously identified in the OCN proximal promoter a minimal C/T-rich motif, termed OCN-CxRE (connexin-response element) that binds Sp1 and Sp3 in a gap junction-dependent manner. In the present study, we hypothesized that Osx could act via this non-canonical Sp1/Sp3-binding element to regulate OCN transcription. OCN promoter luciferase reporter assays show that Osx alone is an insufficient activator that requires Sp1, but not Sp3, to synergistically stimulate OCN promoter activity. Moreover, promoter deletion analyses demonstrate that both the Sp1/Sp3-binding OCN-CxRE (-70 to -57) and the -92 to -87 region of the OCN proximal promoter are critical for Osx/Sp1 synergistic activities. Our data show that Sp1 influences Osx activity by enhancing Osx occupancy on the OCN promoter, perhaps via physical interactions between the two transcription factors. Finally, alteration of the expression of the gap junction protein connexin43 modulates the recruitment of both Sp1 and Osx to the OCN promoter. In total, our data are strongly in support of Sp1 as an essential transcription factor required for Osx recruitment and transactivation of the OCN promoter. Further, these data lend insight into a mechanism by which alteration of connexin43 impacts osteogenesis in vitro and in vivo.

Species-specific regulation of the alpha-2(I) procollagen gene by proximal promoter elements

Transcriptional regulation of the human alpha 2(I) procollagen proximal promoter involves the interaction of trans-acting factors at the inverted CCAAT box (G/CBE) located at position -80 and an adjacent GGAGGCCC-box at -70. Both these elements have previously been shown to be essential for activity of the human promoter. This study investigated nucleotide differences at three sites (-74, -72 and -71) between the human and mouse promoters that were sufficient to abolish trans-acting factor binding with the mouse sequence (GGAGACGT). Two distinct DNA-protein interactions were detected on the human -107/+54 promoter fragment while a single interaction was observed at the equivalent mouse promoter. One of these factors is the CCAAT-binding factor (CBF) and it's binding was observed on both the human and mouse promoters. Although the GGAGGCCC DNA-binding element was not detected on the mouse promoter, GGAGGCC-binding proteins were present in mouse nuclear extracts as observed by their interaction with the human promoter. Functional analysis of the human and mouse -343/+54 and -107/+54 promoter regions revealed significant differences between species; the human constructs having higher activity than the mouse. The differences in promoter activity between species may in part be a result of the nucleotide differences in the GGAGGCCC-box. Mutations in this region of the human -107/+54 promoter prevented DNA-protein interaction and lowered promoter activity. These results support the hypothesis that the GGAGGCCC-box in the human alpha 2(1) procollagen promoter has a regulatory function and that there exists a species-specific difference in transcription factor binding and regulation of the gene.

Dlx3 transcriptional regulation of osteoblast differentiation: temporal recruitment of Msx2, Dlx3, and Dlx5 homeodomain proteins to chromatin of the osteocalcin gene

Genetic studies show that Msx2 and Dlx5 homeodomain (HD) proteins support skeletal development, but null mutation of the closely related Dlx3 gene results in early embryonic lethality. Here we find that expression of Dlx3 in the mouse embryo is associated with new bone formation and regulation of osteoblast differentiation. Dlx3 is expressed in osteoblasts, and overexpression of Dlx3 in osteoprogenitor cells promotes, while specific knock-down of Dlx3 by RNA interference inhibits, induction of osteogenic markers. We characterized gene regulation by Dlx3 in relation to that of Msx2 and Dlx5 during osteoblast differentiation. Chromatin immunoprecipitation assays revealed a molecular switch in HD protein association with the bone-specific osteocalcin (OC) gene. The transcriptionally repressed OC gene was occupied by Msx2 in proliferating osteoblasts, while Dlx3, Dlx5, and Runx2 were recruited postproliferatively to initiate transcription. Dlx5 occupancy increased over Dlx3 in mature osteoblasts at the mineralization stage of differentiation, coincident with increased RNA polymerase II occupancy. Dlx3 protein-DNA interactions stimulated OC promoter activity, while Dlx3-Runx2 protein-protein interaction reduced Runx2-mediated transcription. Deletion analysis showed that the Dlx3 interacting domain of Runx2 is from amino acids 376 to 432, which also include the transcriptionally active subnuclear targeting sequence (376 to 432). Thus, we provide cellular and molecular evidence for Dlx3 in regulating osteoprogenitor cell differentiation and for both positive and negative regulation of gene transcription. We propose that multiple HD proteins in osteoblasts constitute a regulatory network that mediates development of the bone phenotype through the sequential association of distinct HD proteins with promoter regulatory elements.

Tyrosine phosphorylation controls Runx2-mediated subnuclear targeting of YAP to repress transcription

Src/Yes tyrosine kinase signaling contributes to the regulation of bone homeostasis and inhibits osteoblast activity. Here we show that the endogenous Yes-associated protein (YAP), a mediator of Src/Yes signaling, interacts with the native Runx2 protein, an osteoblast-related transcription factor, and suppresses Runx2 transcriptional activity in a dose-dependent manner. Runx2, through its PY motif, recruits YAP to subnuclear domains in situ and to the osteocalcin (OC) gene promoter in vivo. Inhibition of Src/Yes kinase blocks tyrosine phosphorylation of YAP and dissociates endogenous Runx2-YAP complexes. Consequently, recruitment of the YAP co-repressor to subnuclear domains is abrogated and expression of the endogenous OC gene is induced. Our results suggest that Src/Yes signals are integrated through organization of Runx2-YAP transcriptional complexes at subnuclear sites to attenuate skeletal gene expression.

Species-specific mechanisms control the activity of the Pit1/PIT1 phosphate transporter gene promoter in mouse and human

The Pit1 phosphate transporter is involved in regulated phosphate handling in bone forming cells. In this study, we compared the structure of the murine and human Pit1/PIT1 promoters and characterized cis-acting elements controlling Pit1/PIT1 expression. The Pit1/PIT1 promoter sequence and its location relative to the first transcribed exon are conserved and similar transcription factor binding sites are found at identical positions in mouse and human. Luciferase reporter gene assays in transiently transfected mouse ATDC5 chondrocytes and human SaOS-2 osteoblasts indicated that the activity of the mouse Pit1 promoter depends on several cis-acting elements, including ATF/CREB, Sp1 and AP-1 sites, an E-box and a TATA box. In contrast, the activity of the human promoter essentially requires a TATA-like sequence and one single Sp1 site. This Sp1 site binds Sp1, Sp3, as well as unidentified proteins present in SaOS-2 nuclear extracts and co-transfection experiments in SL2 cells indicate that Sp1 and Sp3 activate transcription from the human PIT1 promoter. These data suggest that, despite similarities in promoter structure, changes in the relative importance of conserved transcription factor binding sites cause species-dependent differences in Pit1 promoter function, which allow Sp1-related proteins to play a particularly important role in human.

Integration of FGF and TWIST in calvarial bone and suture development

Mutations in the FGFR1-FGFR3 and TWIST genes are known to cause craniosynostosis, the former by constitutive activation and the latter by haploinsufficiency. Although clinically achieving the same end result, the premature fusion of the calvarial bones, it is not known whether these genes lie in the same or independent pathways during calvarial bone development and later in suture closure. We have previously shown that Fgfr2c is expressed at the osteogenic fronts of the developing calvarial bones and that, when FGF is applied via beads to the osteogenic fronts, suture closure is accelerated (Kim, H.-J., Rice, D. P. C., Kettunen, P. J. and Thesleff, I. (1998) Development 125, 1241–1251). In order to investigate further the role of FGF signalling during mouse calvarial bone and suture development, we have performed detailed expression analysis of the splicing variants of Fgfr1-Fgfr3 and Fgfr4, as well as their potential ligand Fgf2. The IIIc splice variants of Fgfr1-Fgfr3 as well as the IIIb variant of Fgfr2 being expressed by differentiating osteoblasts at the osteogenic fronts (E15). In comparison to Fgf9, Fgf2 showed a more restricted expression pattern being primarily expressed in the sutural mesenchyme between the osteogenic fronts. We also carried out a detailed expression analysis of the helix-loop-helix factors (HLH) Twist and Id1 during calvaria and suture development (E10-P6). Twist and Id1 were expressed by early preosteoblasts, in patterns that overlapped those of the FGF ligands, but as these cells differentiated their expression dramatically decreased. Signalling pathways were further studied in vitro, in E15 mouse calvarial explants. Beads soaked in FGF2 induced Twist and inhibited Bsp, a marker of functioning osteoblasts. Meanwhile, BMP2 upregulated Id1. Id1 is a dominant negative HLH thought to inhibit basic HLH such as Twist. In Drosophila, the FGF receptor FR1 is known to be downstream of Twist. We demonstrated that in Twist(+/)(−) mice, FGFR2 protein expression was altered. We propose a model of osteoblast differentiation integrating Twist and FGF in the same pathway, in which FGF acts both at early and late stages. Disruption of this pathway may lead to craniosynostosis.

Activation of osteocalcin transcription involves interaction of protein kinase A- and protein kinase C-dependent pathways

Osteocalcin is a major noncollagenous protein component of bone extracellular matrix, synthesized and secreted exclusively by osteoblastic cells in the late stage of maturation, and is considered indicator of osteoblast differentiation. Osteocalcin expression is modulated by parathyroid hormone (PTH) and a variety of other factors. The cAMP-dependent protein kinase pathway has been shown previously to have an essential role in PTH signaling and regulation of osteocalcin expression. To determine the extent to which other pathways may also participate in osteocalcin expression, we used rat and human osteoblast-like cell lines to generate stably transfected clones in which the osteocalcin promoter was fused to a luciferase reporter gene. These clones were examined for their responsiveness to agents known to activate or interfere with protein kinase A (PKA)- and protein kinase C (PKC)-dependent pathways. We have found that forskolin, cAMP, and PTH, as well as insulin-like growth factor I (IGF-I) and basic fibroblast growth factor, all were effective in activating the osteocalcin promoter. Phorbol 12-myristate 13-acetate (PMA) was also a strong inducer of the promoter, indicating that PKC plays a role in expression of osteocalcin. In combination with PTH or forskolin, the effect of PMA was additive to synergistic. Calphostin C, a selective inhibitor of PKC, decreased the PMA-, PTH-, and IGF-I-induced luciferase activity in a dose-dependent manner; a PKA inhibitor, H-89, also blocked the induction by PTH and IGF-I but not by PMA. We conclude that regulation of osteocalcin transcription is mediated by both PKA-dependent and PKC-dependent mechanisms and that the respective kinases reside on a linear or convergent pathway.

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.

The osteocalcin gene: a model for multiple parameters of skeletal-specific transcriptional control

Influences of promoter regulatory elements that are responsive to basal and tissue-restricted transactivation factors, steroid hormones, growth factors and other physiologic mediators has provided the basis for understanding regulatory mechanisms contributing to developmental expression of osteocalcin, tissue specificity and biological activity (reviewed in [1-3]). These regulatory elements and cognate transcription factors support postproliferative transcriptional activation and steroid hormone (e.g. vitamin D) enhancement at the onset of extracellular matrix mineralization during osteoblast differentiation. Three parameters of nuclear structure contribute to osteocalcin gene transcriptional control. The linear representation of promoter elements provides competency for physiological responsiveness within the contexts of developmental as well as phenotype-dependent regulation. Chromatin structure and nucleosome organization reduce distances between independent regulatory elements providing a basis for integrating components of transcriptional control. The nuclear matrix supports gene expression by imposing physical constraints on chromatin related to three dimensional genomic organization. In addition, the nuclear matrix facilitates gene localization as well as the concentration and targeting of transcription factors. Several lines of evidence are presented which are consistent with involvement of multiple levels of nuclear architecture in tissue-specific gene expression during differentiation. Growth factor and steroid hormone responsive modifications in chromatin structure, nucleosome organization and the nuclear matrix are considered which influence transcription of the bone tissue-specific osteocalcin gene during progressive expression of the osteoblast phenotype.

Glucocorticoid-dependent transcriptional repression of the osteocalcin gene by competitive binding at the TATA box

The human osteocalcin gene is transcriptionally repressed by glucocorticoids. A specific binding element for the glucocorticoid receptor (GR) overlapping the TATA box of the human osteocalcin promoter has previously been identified. In the present study, the function of this element has been further characterized by competitive gel mobility-shift assay and transfection experiments. The GR and TATA-binding protein (TBP) bound to the cognate overlapping elements in a mutually exclusive manner. The GR preferentially inhibited the binding of TBP. The isolated DNA-binding domain of the GR is sufficient to compete for TBP binding. The integrity of both half-sites of the glucocorticoid response element (GRE) is required to effectively compete for TBP binding, and competitive binding of the GR is dependent on dimerization. Transient overexpression of TBP overrides the transcriptional repression of the osteocalcin promoter by glucocorticoids. We conclude that the repressive effect of glucocorticoids on this promoter is the result of competitive DNA binding to a basal transcriptional element and that it does not appear to require direct protein-protein interaction between the competitive factors.

Bone tissue-specific transcription of the osteocalcin gene: role of an activator osteoblast-specific complex and suppressor hox proteins that bind the OC box

Bone-specific expression of the osteocalcin gene is transcriptionally controlled. Deletion analysis of osteocalcin promoter sequences by transient transfection of osseous (ROS 17/2.8) and nonosseous (R2 fibroblast) cells revealed that the most proximal 108 nucleotides are sufficient to confer tissue-specific expression. By gel mobility shift assays with wild-type and mutated oligonucleotides and nuclear extracts from several different cell lines we identified a novel transcription factor complex which exhibits sequence-specific interactions with the primary transcriptional element, the OC box (nt -99 to -76). This OC box binding protein (OCBP) is present only in osteoblast-like cells. Methylation interference demonstrated association of the factor with OC box sequences overlapping the Msx homeodomain consensus binding site. By assaying several mutations of the OC box, both in gel shift and transient transfection studies using ROS 17/2.8, we show the following. First, binding of OCBP correlates with osteocalcin promoter activity in ROS 17/2.8 cells. Increased binding leads to a 2-3-fold increase in transcription, while decreased binding results in transcription 30-40% of control. Second, homeodomain protein binding suppresses transcription. However, Msx expression is critical for full development of the bone phenotype as determined by antisense studies. Last, we show that one of the mutations of the OC box permits expression of osteocalcin in non-osseous cell lines. In summary, we demonstrate association of at least two classes of tissue-restricted transcription factors with the OC box element, the OCBP and Msx proteins, supporting the concept that these sequences contribute to defining tissue specificity.

Contributions of nuclear architecture to transcriptional control

Three parameters of nuclear structure contribute to transcriptional control. The linear representation of promoter elements provides competency for physiological responsiveness within the contexts of development as well as cycle- and phenotype-dependent regulation. Chromatin structure and nucleosome organization reduce distances between independent regulatory elements providing a basis for integrating components of transcriptional control. The nuclear matrix supports gene expression by imposing physical constraints on chromatin related to three-dimensional genomic organization. In addition, the nuclear matrix facilitates gene localization as well as the concentration and targeting of transcription factors. Several lines of evidence are presented that are consistent with involvement of multiple levels of nuclear architecture in cell growth and tissue-specific gene expression during differentiation. Growth factor and steroid hormone responsive modifications in chromatin structure, nucleosome organization, and the nuclear matrix that influence transcription of the cell cycle-regulated histone gene and the bone tissue-specific osteocalcin gene during progressive expression of the osteoblast phenotype are considered.

G/C element contributes to the cell line-specific expression of the proximal osteocalcin promoter

Sequential activation of cell type-specific genes occurs during osteoblast development. The promoter of one such gene, osteocalcin, has been widely studied, but the DNA sequences that govern osteoblast-specific expression have not been defined. The proximal osteocalcin promoter linked to pTKCAT directs strong promoter activity in osteoblast-like ROS17/2.8 cells and comparatively weak promoter activity in nonosteoblastic NIH3T3 cells. To identify sequences important in conferring cell-specific expression of the osteocalcin gene, a deletion series of the human proximal promoter was constructed and the activities assessed in ROS17/2.8 and NIH3T3 cells. These studies identified a 30 bp sequence within the proximal promoter (osteocalcin repressor element-1 [ORE-1]) which is responsible for repressing the transcriptional activity in NIH3T3 cells. In electrophoretic mobility shift assays from both NIH3T3 and ROS17/2.8 cells, a protein complex bound to the ORE-1 that was related to a complex which binds the G/C-rich repressor element in the collagen type I (alpha 1) promoter. In addition, there was a second complex from NIH3T3 cells but not ROS17/2.8 cells that bound the ORE-1 fragment. The presence of this additional factor in NIH3T3 cells parallels the observation that constructs carrying the ORE-1 sequence have repressed promoter activity relative to the analogous constructs lacking the ORE-1 when transfected into NIH3T3 and suggests that the NIH3T3-specific factor is a repressor. These data indicate that the G/C element in the ORE-1 contributes to the repression of osteocalcin gene transcription in a nonosteoblast cell line. The high homology between the ORE-1 sequence and a related sequence in the collagen type I (alpha 2) proximal promoter suggests that homologous regions in other osteoblast-expressed genes may function similarly.

The osteocalcin and collagen type I (alpha 1) promoters share common basal regulatory units

Sequential activation of osteoblast-specific genes occurs during cell development. Regulation of these genes is through the cooperation between basal, hormone-responsive, and growth factor-responsive transcriptional control elements. The active hormone, 1,25-dihydroxyvitamin D3 plays an important role in the regulation of osteocalcin and other osteoblast-expressed genes. As well as containing a vitamin D response element, the upstream region of the osteocalcin promoter also has potent basal activity in the osteoblast-like ROS17/2.8 cell line. The present study identifies a short DNA sequence that contributes to basal promoter activity. This osteocalcin cis-acting response element (OSCARE-1) has two basal regulatory elements: a G/C-rich element and an adjacent reverse CCAAT element. Homologous sequences have been characterized as negative and positive basal regulatory elements, respectively, in the promoter of the collagen type I (alpha 1) gene. In electrophoretic mobility-shift assays, this collagen regulatory unit and OSCARE-1 produce similar banding patterns and bind common ROS17/2.8 nuclear proteins. Mutations of the G/C element in the collagen promoter showed that it functions as an inhibitory element in NIH-3T3 cells. Introduction of the same mutations into the G/C element of the OSCARE-1 unit exposed a similar repressive activity in NIH-3T3 cells, which correlated with an altered electrophoretic mobility-shift assay banding pattern. We have shown a similarity between a basal regulatory unit in the distal osteocalcin promoter and a unit in the proximal collagen type I (alpha 1) promoter. The fact that similar units are present in other osteoblast-specific promoters suggests that OSCARE-1-like units may be a common regulator of osteoblast-expressed genes.

Proximal promoter binding protein contributes to developmental, tissue-restricted expression of the rat osteocalcin gene

Osteocalcin is a 6 kD tissue-specific calcium binding protein associated with the bone extracellular matrix. The osteocalcin gene is developmentally expressed in postoproliferative rat osteoblasts with regulation at least in part at the transcriptional level. Multiple, basal promoter and enhancer elements which control transcriptional activity in response to physiological mediators, including steroid hormones, have been identified in the modularly organized osteocalcin gene promoter. The osteocalcin box (OC box) is a highly conserved basal regulatory element residing between nucleotides -99 and -76 of the proximal promoter. We recently established by in vivo competition analysis that protein interactions at the CCAAT motif, which is the central core of the rat OC box, are required for support of basal transcription [Heinrichs et al. J Cell Biochem 53:240-250, 1993]. In this study, by the combined utilization of electrophoretic mobility shift analysis, UV cross linking, and DNA affinity chromatography, we have identified a protein that binds to the rat OC box. Results are presented that support involvement of the OC box-binding protein in regulating selective expression of the osteocalcin gene during differentiation of the rat osteoblast phenotype and suggest that this protein is tissue restricted.

Transcriptional control of the tissue-specific, developmentally regulated osteocalcin gene requires a binding motif for the Msx family of homeodomain proteins

The OC box of the rat osteocalcin promoter (nt -99 to -76) is the principal proximal regulatory element contributing to both tissue-specific and developmental control of osteocalcin gene expression. The central motif of the OC box includes a perfect consensus DNA binding site for certain homeodomain proteins. Homeodomain proteins are transcription factors that direct proper development by regulating specific temporal and spatial patterns of gene expression. We therefore addressed the role of the homeodomain binding motif in the activity of the OC promoter. In this study, by the combined application of mutagenesis and site-specific protein recognition analysis, we examined interactions of ROS 17/2.8 osteosarcoma cell nuclear proteins and purified Msx-1 homeodomain protein with the OC box. We detected a series of related specific protein-DNA interactions, a subset of which were inhibited by antibodies directed against the Msx-1 homeodomain but which also recognize the Msx-2 homeodomain. Our results show that the sequence requirements for binding the Msx-1 or Msx-2 homeodomain closely parallel those necessary for osteocalcin gene promoter activity in vivo. This functional relationship was demonstrated by transient expression in ROS 17/2.8 osteosarcoma cells of a series of osteocalcin promoter (nt -1097 to +24)-reporter gene constructs containing mutations within and flanking the homeodomain binding site of the OC box. Northern blot analysis of several bone-related cell types showed that all of the cells expressed msx-1, whereas msx-2 expression was restricted to cells transcribing osteocalcin. Taken together, our results suggest a role for Msx-1 and -2 or related homeodomain proteins in transcription of the osteocalcin gene.

A composite intragenic silencer domain exhibits negative and positive transcriptional control of the bone-specific osteocalcin gene: promoter and cell type requirements

The osteocalcin (OC) silencer is a unique example of exonic sequences contributing to negative transcriptional control of mammalian gene expression. In this paper we demonstrate, using a reporter transfection assay, that multiple elements reside within the OC +24/+151 domain. Thirty-fold repression is mediated by the +49/+104 fragment, experimentally relocated 3' of the poly(A) signal. Deletion of either the +49/+54 protein-coding sequence or the +98/+104 intronic part of this fragment results in loss of repression activity, suggesting a bipartite organization of the +49/+104 silencer. Of particular interest, we have mapped an antisilencer activity to the ACCCTCTCT motif (+40/+48), found in silencers associated with several other genes. Extension of the +49/+104 silencer to include the +24/+48 and/or the +105/+151 sequences results in increased silencer activity up to 170-fold, suggesting the presence of additional silencer elements within these sequences. The activity of the silencer contained within the +24/+151 OC sequence is directed to the basal promoter and is not dependent on 5' distal enhancer elements, including those that mediate responsiveness of OC transcription to vitamin D. The OC silencer represses the heterologous thymidine kinase promoter and is operative in osseous (normal diploid osteoblasts, ROS 17/2.8 osteosarcoma) as well as HeLa cells. Our results, which suggest the presence of at least five regulatory elements downstream of the OC transcription start site, indicate the complexity of sequences that mediate repression of OC promoter activity.

DNase I hypersensitive sites in promoter elements associated with basal and vitamin D dependent transcription of the bone-specific osteocalcin gene

Nuclease hypersensitive sites were mapped in the proximal promoter of the osteocalcin gene, which is expressed only in bone cells exhibiting the mature osteoblast phenotype. Nuclei from proliferating and confluent rat osteosarcoma (ROS) 17/2.8 cells were subjected to DNase I digestion, and hypersensitivity was assayed by the indirect end-labeling method, using osteocalcin gene probes. Hypersensitive sites were detected in two promoter domains: -590 to -390, which spans the vitamin D responsive element, and -170 to -70, which spans the TATA box and the CCAAT-containing OC box domain. Together, these elements regulate basal and vitamin D enhanced osteocalcin gene transcription. We observed a parallel relationship between the intensity of bands representing the hypersensitive sites and the extent to which the osteocalcin gene is transcribed. Both in confluent cultures and in response to vitamin D, when osteocalcin transcription was upregulated, the hypersensitive bands were significantly intensified. Additionally, the bands were decreased under conditions that downregulate osteocalcin gene transcription. A functional relationship between the presence of hypersensitive sites and osteocalcin gene transcription is further supported by the absence of hypersensitivity in nonosseous cells that do not express osteocalcin, although these proliferating cells exhibited hypersensitivity in a cell cycle regulated histone gene promoter. Our results suggest the involvement of chromatin structure in transcriptional responsiveness of the osteocalcin gene to physiologic modulation.

Position and orientation-selective silencer in protein-coding sequences of the rat osteocalcin gene

Osteocalcin (OC) is a bone-specific protein which is expressed postproliferatively by osteoblasts during late stages of differentiation. We have found that a silencer element is present within the rat OC gene (between nt +39 and +104), overlapping the OC signal prepropeptide-coding sequence. The presence of this sequence in OC promoter-CAT reporter constructs suppresses promoter activity in transiently transfected proliferating osteoblasts, which do not express OC, by up to 50-fold. This is the first demonstration of contribution from protein-coding sequences to silencing of animal genes. The element appears to be bipartite; silencer activity requires both the protein-coding sequence +39 to +63 and the +93 to +104 exon 1/intron 1 border region. Both of these domains contain sequences highly similar to silencer motifs in several other genes, including chicken lysozyme as well as rat collagen type II, insulin, and growth hormone. OC silencer activity is fully retained when the element is placed outside the RNA-coding region, 3' but not 5' of the OC-CAT fusion gene. Repression activity is orientation independent in the native position but requires the native orientation when located in 3' extragenic positions. The silencer does not inhibit the activity of the heterologous SV40 early promoter. These results suggest interaction between the transcribed silencer and specific OC promoter element(s) residing farther upstream. The OC transcribed silencer may contribute to developmental control of OC expression.