Characterization of an osteoblast-specific enhancer element in the CBFA1 gene

Crosstalk between protein kinase C α and transforming growth factor β signaling mediated by Runx2 in intestinal epithelial cells

Tight coordination of growth regulatory signaling is required for intestinal epithelial homeostasis. Protein kinase C α (PKCα) and transforming growth factor β (TGFβ) are negative regulators of proliferation with tumor suppressor properties in the intestine. Here, we identify novel crosstalk between PKCα and TGFβ signaling. RNA-Seq analysis of nontransformed intestinal crypt-like cells and colorectal cancer cells identified TGFβ receptor 1 (TGFβR1) as a target of PKCα signaling. RT-PCR and immunoblot analysis confirmed that PKCα positively regulates TGFβR1 mRNA and protein expression in these cells. Effects on TGFβR1 were dependent on Ras-extracellular signal-regulated kinase 1/2 (ERK) signaling. Nascent RNA and promoter-reporter analysis indicated that PKCα induces TGFβR1 transcription, and Runx2 was identified as an essential mediator of the effect. PKCα promoted ERK-mediated activating phosphorylation of Runx2, which preceded transcriptional activation of the TGFβR1 gene and induction of Runx2 expression. Thus, we have identified a novel PKCα→ERK→Runx2→TGFβR1 signaling axis. In further support of a link between PKCα and TGFβ signaling, PKCα knockdown reduced the ability of TGFβ to induce SMAD2 phosphorylation and cell cycle arrest, and inhibition of TGFβR1 decreased PKCα-induced upregulation of p21^Cip1 and p27^Kip1 in intestinal cells. The physiological relevance of these findings is also supported by The Cancer Genome Atlas data showing correlation between PKCα, Runx2, and TGFβR1 mRNA expression in human colorectal cancer. PKCα also regulated TGFβR1 in endometrial cancer cells, and PKCα, Runx2, and TGFβR1 expression correlates in uterine tumors, indicating that crosstalk between PKCα and TGFβ signaling may be a common mechanism in diverse epithelial tissues.

Dysregulation of circRNA-0076906 and circRNA-0134944 is Correlated with Susceptibility to Osteoporosis and Osteoporotic Fracture in Postmenopausal Females from the Chinese Han Population

Introduction

Many circRNAs, such as circRNA-0076906 and circRNA-0134944, have been reported to participate in the pathogenesis of osteoporosis via sponging miRNAs in postmenopausal female patients. In this study, we aimed to study potential signaling pathways underlying the role of certain circRNAs, miRNAs and their target genes in the pathogenesis of osteoporotic fracture in postmenopausal females.

Methods

Quantitative real-time PCR was performed to analyze the expression of circRNAs, miRNAs and their targets genes. Luciferase assays were carried out to explore the regulatory relationship between circ_0076906/miR-548i/OGN and circ_0134944/miR-630/TLR4.

Results

Osteoporosis and fracture were positively correlated to the expression of circ_0134944, miR-548i and TLR4, but negatively correlated to the expression of circ_0076906, miR-630 and OGN in the peripheral blood and bone tissue samples of postmenopausal women. Luciferase activities of wild-type circ_0076906 and OGN were inhibited by miR-548i, and the luciferase activities of wild-type circ_0134944 and TLR4 were suppressed by miR-630 in MG-63 and U-2 OS cells. Inhibition of circ_0076906 expression in MG-63 and U-2 OS cells activated the expression of miR-548i and inhibited the expression of OGN. Moreover, the overexpression of circ_0134944 in MG-63 and U-2 OS cells suppressed the expression of miR-630 and enhanced the expression of TLR4.

Conclusion

This study implied that the dysregulation of circRNA-0076906 and circRNA-0134944 modulated their specific signaling and thus contributed to the severity of osteoporosis, increasing the risk of osteoporotic fracture.

An miRNA derived from amelogenin exon4 regulates expression of transcription factor Runx2 by directly targeting upstream activators Nfia and Prkch

Amel, the gene encoding the amelogenin protein involved in enamel formation, is highly alternatively spliced. When exon4 is excised, it can form a mature miRNA (miR-exon4) that has previously been suggested to indirectly regulate expression of the Runt-related transcription factor 2 (Runx2) involved in bone development in ameloblasts and osteoblasts. However, the precise mechanism of this regulation is unclear. In this study, we aimed to identify direct targets of miR-exon4. The transcription factor family nuclear factor I/A (NFI/A) is known to negatively regulate expression of Runx2 and is among the most highly predicted direct targets of miR-exon4 that link to Runx2. Immunostaining detected NFI/A in osteoblasts and ameloblasts in vivo, and reporter assays confirmed direct interaction of the Nfia 3'-UTR and miR-exon4. In addition, silencing of Nfia in MC3T3-E1-M14 osteoblasts resulted in subsequent downregulation of Runx2. In a monoclonal subclone (mi2) of MC3T3-E1 cells wherein mature miR-exon4 was functionally inhibited, we observed significantly downregulated Runx2 expression. We showed that NFI/A was significantly upregulated in mi2 cells at both mRNA and protein levels. Furthermore, quantitative proteomics and pathway analysis of gene expression in mi2 cells suggested that miR-exon4 could directly target Prkch (protein kinase C-eta), possibly leading to RUNX2 regulation through mechanistic target of rapamycin kinase activation. Reporter assays also confirmed the direct interaction of miR-exon4 and the 3'-UTR of Prkch, and Western blot analysis confirmed significantly upregulated mechanistic target of rapamycin kinase phosphorylation in mi2 cells. Taken together, we conclude that Nfia and Prkch expression negatively correlates with miR-exon4-mediated Runx2 regulation in vivo and in vitro, suggesting miR-exon4 directly targets Nfia and Prkch to regulate Runx2.

Circ_0087960 stabilizes KDM5B by reducing SKP2 mediated ubiquitination degradation and promotes osteogenic differentiation in periodontal ligament stem cells

Background

Periodontitis is a common chronic oral disease among the world. Periodontal ligament stem cells (PDLSCs) has been proved to be a promising tool for the treatment of periodontitis due to their capability of generating periodontal tissues. Circ_0087960 and KDM5B have been shown to participate in the process of osteogenic differentiation with unclear function and mechanism.

Methods

Circ_0087960 and KDM5B expressions were detected during the osteogenic induction of PDLSCs. The functions of circ_0087960 and KDM5B were validated by manipulating their expression with shRNA. ChIP and luciferase reporter assays were used to prove the KDM5B-based osteogenic gene regulation. Co-IP assay was used to determine the interaction between SKP2 and KDM5B. In vivo ubiquitination assay was used to test the modification of KDM5B by SKP2. RNA pull-down was used to demonstrate the interaction between circ_0087960 and KDM5B.

Results

Circ_0087960 and KDM5B were found to be upregulated in the osteogenic differentiation of PDLSCs and promote the expression of related genes. KDM5B could directly bind and promote the expression of Runx2, ALP and OCN. KDM5B protein level in PDLSCs was controlled by SKP2-mediated protein ubiquitination and degradation. Circ_0087960 was identified to bind to KDM5B protein and protect it against SKP2-induced protein degradation, leading to the upregulation of osteogenic genes.

Conclusion

Circ_0087960 and KDM5B could be applied as promising therapeutic methods to stimulate the osteogenic differentiation of PDLSCs, expanding their capability in the treatment of periodontitis.

Transcriptional Programming in Arteriosclerotic Disease: A Multifaceted Function of the Runx2 (Runt-Related Transcription Factor 2)

Despite successful therapeutic strategies in the prevention and treatment of arteriosclerosis, the cardiovascular complications remain a major clinical and societal issue worldwide. Increased vascular calcification promotes arterial stiffness and accelerates cardiovascular morbidity and mortality. Upregulation of the Runx2 (Runt-related transcription factor 2), an essential osteogenic transcription factor for bone formation, in the cardiovascular system has emerged as an important regulator for adverse cellular events that drive cardiovascular pathology. This review discusses the regulatory mechanisms that are critical for Runx2 expression and function and highlights the dynamic and complex cross talks of a wide variety of posttranslational modifications, including phosphorylation, acetylation, ubiquitination, and O-linked β-N-acetylglucosamine modification, in regulating Runx2 stability, cellular localization, and osteogenic transcriptional activity. How the activation of an array of signaling cascades by circulating and local microenvironmental factors upregulates Runx2 in vascular cells and promotes Runx2-mediated osteogenic transdifferentiation of vascular smooth muscle cells and expression of inflammatory cytokines that accelerate macrophage infiltration and vascular osteoclast formation is summarized. Furthermore, the increasing appreciation of a new role of Runx2 upregulation in promoting vascular smooth muscle cell phenotypic switch, and Runx2 modulated by O-linked β-N-acetylglucosamine modification and Runx2-dependent repression of smooth muscle cell-specific gene expression are discussed. Further exploring the regulation of this key osteogenic transcription factor and its new perspectives in the vasculature will provide novel insights into the transcriptional regulation of vascular smooth muscle cell phenotype switch, reprograming, and vascular inflammation that promote the pathogenesis of arteriosclerosis.

Transcription Factor HOXA9 is Linked to the Calcification and Invasion of Papillary Thyroid Carcinoma

Calcification is important for the diagnosis of papillary thyroid carcinoma (PTC). Runt-related transcription factor 2 (RUNX2), a master transcription factor associated with osteogenic differentiation, is reportedly related to PTC calcification and invasiveness. However, its regulatory role in this process is somewhat uncharacterized. Here, we attempted to identify genes that regulate RUNX2 and clarify its function in PTC carcinogenesis and calcification. The expression of RUNX2-upstream genes was evaluated by real-time PCR in Nthy-Ori 3-1 normal thyroid cells and TPC1 and BHP10-3 PTC cell lines. Luciferase and chromatin immunoprecipitation assays were performed with candidate genes after cloning the RUNX2 promoter. We found that RUNX2 promoter activity was enhanced by homeobox family A9 (HOXA9). Over-expression of HOXA9 was found to enhance alkaline phosphatase activity, mineralization, and in vitro tumour cell migration and invasion, whereas downregulation had the opposite effects. These results indicate that HOXA9, a positive regulator of RUNX2, can enhance calcification, migration, and invasion in PTC. Our data improve the understanding of the molecular mechanisms of microcalcification in PTC as well as tumorigenesis.

Effects of Osteoglycin (OGN) on treating senile osteoporosis by regulating MSCs

Background

Significant amount of bone mass is lost during the process of aging due to an imbalance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption in bone marrow microenvironment, which leads to net bone loss in the aging population, resulting in the pathogenesis of osteoporosis.

Methods

Firstly, differences in proliferative capacity of adipocyte or adipogenic differentiation in mouse mesenchymal stem cells (MMSCs) and senile mouse model-derived bone marrow mesenchymal stem cells (SMMSCs), as well as mRNA expression of OGN and PPARγ2 were observed. Secondly, osteogenic abilities of MMSCs and SMMSCs treated with rosiglitazone (a PPARγ2 agonist) to induce osteogenic changes were observed, and negative correlation of PPARγ2 with OGN was evaluated. Thirdly, the role of SMMSCs in promoting osteogenesis was examined through enhancing expression of OGN; besides, the related mechanism was investigated by means of expression of related adipocyte and osteoblast specific genes.

Results

Forced OGN expression by OGN-infected lentivirus could increase expression of Wnt5b, RUNX2, OCN, ALP and Colla1, as well as bone formation, while decreases expression of adipogenesis marker PPARγ2. It resulted in expression inhibition of adipocyte genes such as adipocytic differentiation related genes adipocyte binding protein 2 (aP2) and osteoclast differentiation factor Rankl in bone marrow, giving rise to increased bone mass.

Conclusion

OGN may plays a significant role in osteoporosis, which may also provide a potential target for therapeutic intervention of senile osteoporosis characterized by altered differentiation of BMSCs into osteoblasts and adipocytes.

Roles of Runx2 in Skeletal Development

Runx2 is the most upstream transcription factor essential for osteoblast differentiation. It regulates the expression of Sp7, the protein of which is a crucial transcription factor for osteoblast differentiation, as well as that of bone matrix genes including Spp1, Ibsp, and Bglap2. Runx2 is also required for chondrocyte maturation, and Runx3 has a redundant function with Runx2 in chondrocyte maturation. Runx2 regulates the expression of Col10a1, Spp1, Ibsp, and Mmp13 in chondrocytes. It also inhibits chondrocytes from acquiring the phenotypes of permanent cartilage chondrocytes. It regulates chondrocyte proliferation through the regulation of Ihh expression. Runx2 enhances osteoclastogenesis by regulating Rankl. Cbfb, which is a co-transcription factor for Runx family proteins, plays an important role in skeletal development by stabilizing Runx family proteins. In Cbfb isoforms, Cbfb1 is more potent than Cbfb2 in Runx2-dependent transcriptional regulation; however, the expression level of Cbfb2 is three-fold higher than that of Cbfb1, demonstrating the requirement of Cbfb2 in skeletal development. The expression of Runx2 in osteoblasts is regulated by a 343-bp enhancer located upstream of the P1 promoter. This enhancer is activated by an enhanceosome composed of Dlx5/6, Mef2, Tcf7, Ctnnb1, Sox5/6, Smad1, and Sp7. Thus, Runx2 is a multifunctional transcription factor that is essential for skeletal development, and Cbfb regulates skeletal development by modulating the stability and transcriptional activity of Runx family proteins.

Epigenetic Control of the Bone-master Runx2 Gene during Osteoblast-lineage Commitment by the Histone Demethylase JARID1B/KDM5B

Transcription factor Runx2 controls bone development and osteoblast differentiation by regulating expression of a significant number of bone-related target genes. Here, we report that transcriptional activation and repression of the Runx2 gene via its osteoblast-specific P1 promoter (encoding mRNA for the Runx2/p57 isoform) is accompanied by selective deposition and elimination of histone marks during differentiation of mesenchymal cells to the osteogenic and myoblastic lineages. These epigenetic profiles are mediated by key components of the Trithorax/COMPASS-like and Polycomb group complexes together with histone arginine methylases like PRMT5 and lysine demethylases like JARID1B/KDM5B. Importantly, knockdown of the H3K4me2/3 demethylase JARID1B, but not of the demethylases UTX and NO66, prevents repression of the Runx2 P1 promoter during myogenic differentiation of mesenchymal cells. The epigenetically forced expression of Runx2/p57 and osteocalcin, a classical bone-related target gene, under myoblastic-differentiation is accompanied by enrichment of the H3K4me3 and H3K27ac marks at the Runx2 P1 promoter region. Our results identify JARID1B as a key component of a potent epigenetic switch that controls mesenchymal cell fate into myogenic and osteogenic lineages.

Role of RUNX2 in Breast Carcinogenesis

RUNX2 is a transcription factor playing the major role in osteogenesis, but it can be involved in DNA damage response, which is crucial for cancer transformation. RUNX2 can interact with cell cycle regulators: cyclin-dependent kinases, pRB and p21Cip1 proteins, as well as the master regulator of the cell cycle, the p53 tumor suppressor. RUNX2 is involved in many signaling pathways, including those important for estrogen signaling, which, in turn, are significant for breast carcinogenesis. RUNX2 can promote breast cancer development through Wnt and Tgfβ signaling pathways, especially in estrogen receptor (ER)-negative cases. ERα interacts directly with RUNX2 and regulates its activity. Moreover, the ERα gene has a RUNX2 binding site within its promoter. RUNX2 stimulates the expression of aromatase, an estrogen producing enzyme, increasing the level of estrogens, which in turn stimulate cell proliferation and replication errors, which can be turned into carcinogenic mutations. Exploring the role of RUNX2 in the pathogenesis of breast cancer can lead to revealing new therapeutic targets.

Loss of Tbx1 induces bone phenotypes similar to cleidocranial dysplasia

T-box transcription factor, TBX1, is the major candidate gene for 22q11.2 deletion syndrome (DiGeorge/ Velo-cardio-facial syndrome) characterized by facial defects, thymus hypoplasia, cardiovascular anomalies and cleft palates. Here, we report that the loss of Tbx1 in mouse (Tbx1(-/-)) results in skeletal abnormalities similar to those of cleidocranial dysplasia (CCD) in humans, which is an autosomal-dominant skeletal disease caused by mutations in RUNX2. Tbx1(-/-) mice display short stature, absence of hyoid bone, failed closure of fontanelle, bifid xiphoid process and hypoplasia of clavicle and zygomatic arch. A cell-type-specific deletion of Tbx1 in osteochondro-progenitor (Tbx1(OPKO)) or mesodermal (Tbx1(MKO)) lineage partially recapitulates the Tbx1(-/-) bone phenotypes. Although Tbx1 expression has not been previously reported in neural crest, inactivation of Tbx1 in the neural crest lineage (Tbx1(NCKO)) leads to an absence of the body of hyoid bone and postnatal lethality, indicating an unanticipated role of Tbx1 in neural crest development. Indeed, Tbx1 is expressed in the neural crest-derived hyoid bone primordium, in addition to mesoderm-derived osteochondral progenitors. Ablation of Tbx1 affected Runx2 expression in calvarial bones and overexpression of Tbx1 induced Runx2 expression in vitro. Taken together, our current studies reveal that Tbx1 is required for mesoderm- and neural crest-derived osteoblast differentiation and normal skeletal development. TBX1 mutation could lead to CCD-like bone phenotypes in human.

Dlx5 and mef2 regulate a novel runx2 enhancer for osteoblast-specific expression

Runx2 is essential for osteoblast differentiation and chondrocyte maturation. The expression of Runx2 is the first requisite step for the lineage determination from mesenchymal stem cells to osteoblasts. Although the transcript from Runx2 distal promoter is majorly expressed in osteoblasts, the promoter failed to direct green fluorescent protein (GFP) expression to osteoblasts. To find the regulatory region, we generated GFP reporter mice driven by a bacterial artificial chromosome (BAC) of Runx2 locus, and succeeded in the reproduction of endogenous Runx2 expression. By serially deleting it, we identified a 343-bp enhancer, which directed GFP expression specifically to osteoblasts, about 30 kb upstream of the distal promoter. The sequence of the 343-bp enhancer was highly conserved among mouse, human, dog, horse, opossum, and chicken. Dlx5, Mef2c, Tcf7, Ctnnb1, Sp7, Smad1, and Sox6, which localized on the enhancer region in primary osteoblasts, synergistically upregulated the enhancer activity, whereas Msx2 downregulated the activity in mouse osteoblastic MC3T3-E1 cells. Msx2 was predominantly bound to the enhancer in mouse multipotent mesenchymal C3H10T1/2 cells, whereas Dlx5 was predominantly bound to the enhancer in MC3T3-E1 cells. Dlx5 and Mef2 directly bound to the enhancer, and the binding sites were required for the osteoblast-specific expression in mice, whereas the other factors bound to the enhancer by protein-protein interaction. The enhancer was characterized by the presence of the histone variant H2A.Z, the enrichment of histone H3 mono- and dimethylated at Lys4 and acetylated at Lys18 and Lys27, but the depletion of histone H3 trimethylated at Lys4 in primary osteoblasts. These findings indicated that the enhancer, which had typical histone modifications for enhancers, contains sufficient elements to direct Runx2 expression to osteoblasts, and that Dlx5 and Mef2, which formed an enhanceosome with Tcf7, Ctnnb1, Sp7, Smad1, and Sox6, play an essential role in the osteoblast-specific activation of the enhancer. © 2014 American Society for Bone and Mineral Research.

Epigenetic landscape during osteoblastogenesis defines a differentiation-dependent Runx2 promoter region

Runx2 is a developmentally regulated gene in vertebrates and is essential for bone formation and skeletal homeostasis. The induction of runx2-P1 isoform transcripts is a hallmark of early osteoblastogenesis. Although previous in vitro studies have defined a minimal Runx2-P1 promoter sequence with well-characterized functional elements, several lines of evidence suggest that transcription of the Runx2-P1 isoform relies on elements that extend beyond the previously defined P1 promoter boundaries. In this study, we examined Runx2-P1 transcriptional regulation in a cellular in vivo context during early osteoblastogenesis of MC3T3-E1 cultures and BMSCs induced towards the bone lineage by multi-layered analysis of the Runx2-P1 gene promoter using the following methodologies: 1) sequence homology among several mammalian species, 2) DNaseI hypersensitivity coupled with massively parallel sequencing (DNase-seq), and 3) chromatin immunoprecipitation of activating histone modifications coupled with massively parallel sequencing (ChIP-seq). These epigenetic features have allowed the demarcation of boundaries that redefine the minimal Runx2-P1 promoter to include a 336-bp sequence that mediates responsiveness to osteoblast differentiation. We also find that an additional level of control is contributed by a regulatory region in the 5'-UTR of Runx2-P1.

Genomic occupancy of HLH, AP1 and Runx2 motifs within a nuclease sensitive site of the Runx2 gene

Epigenetic mechanisms mediating expression of the Runt-related transcription factor Runx2 are critical for controlling its osteogenic activity during skeletal development. Here, we characterized bona fide regulatory elements within 120 kbp of the endogenous bone-related Runx2 promoter (P1) in osteoblasts by genomic DNase I footprinting and chromatin immuno-precipitations (ChIPs). We identified a ~10 kbp genomic domain spanning the P1 promoter that interacts with acetylated histones H3 and H4 reflecting an open chromatin conformation in MC3T3 osteoblasts. This large chromatin domain contains a single major DNaseI hypersensitive (DHS) region that defines a 0.4 kbp "basal core" promoter. This region encompasses two endogenous genomic protein/DNA interaction sites (i.e., footprints at Activating Protein 1 [AP1], E-box and Runx motifs). Helix-Loop-Helix (HLH)/E-box occupancy and presence of the DHS region persists in several mesenchymal cell types, but AP1 site occupancy occurs only during S phase when Runx2 expression is minimal. Point-mutation of the HLH/E box dramatically reduces basal promoter activity. Our results indicate that the Runx2 P1 promoter utilizes two stable principal protein/DNA interaction domains associated with AP1 and HLH factors. These sites function together with dynamic and developmentally responsive sites in a major DHS region to support epigenetic control of bone-specific transcription when osteoblasts transition into a quiescent or differentiated state.

The transcription factor paired box-5 promotes osteoblastogenesis through direct induction of Osterix and Osteocalcin

Although skeletal abnormalities are seen in mice deficient of particular paired box (Pax) family proteins, little attention has been paid to their role in osteoblastogenesis so far. Here, we investigated the possible involvement of several Pax family members in mechanisms underlying the regulation of differentiation and maturation of osteoblasts. Among different Pax family members tested, Pax5 was not markedly expressed in murine calvarial osteoblasts before culture, but progressively expressed by osteoblasts under differentiation toward maturation. Immunoreactive Pax5 was highly detectable in primary cultured mature osteoblasts on immunoblotting and in osteoblastic cells attached to cancellous bone in mouse tibial sections on immunohistochemistry, respectively. Knockdown by small interfering RNA (siRNA) of endogenous Pax5 led to significant inhibition of the expression of Osteocalcin, and Osterix through deterioration of gene transactivation, in addition to a1(I)Collagen expression and alkaline phosphatase (ALP) staining, without affecting runt-related transcription factor-2 (Runx2) expression and cell viability in osteoblastic MC3T3-E1 cells. The introduction of Pax5 enhanced promoter activities of Osteocalcin and Osterix in a manner dependent on the paired domain in MC3T3-E1 cells. Putative Pax5 binding sites were identified in the 5'-flanking regions of mouse Osteocalcin and Osterix, whereas chromatin immunoprecipitation assay revealed the direct binding of Pax5 to particular regions of Osteocalcin and Osterix promoters in MC3T3-E1 cells. Overexpression of Pax5 significantly increased Osteocalcin, Osterix, and a1(I)Collagen expression, ALP activity, and Ca(2+) accumulation, without affecting Runx2 expression, in MC3T3-E1 cells. In vertebrae of transgenic mice predominantly expressing Pax5 in osteoblasts, a significant increase was seen in the ratio of bone volume over tissue volume and the bone formation rate. These findings suggest that Pax5 could positively regulate osteoblastic differentiation toward maturation in vitro, in addition to promoting bone formation and remodeling in vivo, as one of the transcription factors essential for controlling osteoblastogenesis independently of Runx2.

Negative regulation of osteoblastogenesis through downregulation of runt-related transcription factor-2 in osteoblastic MC3T3-E1 cells with stable overexpression of the cystine/glutamate antiporter xCT subunit

We have previously demonstrated that glutamate (Glu) suppresses cellular proliferation toward self-renewal through a mechanism associated with intracellular GSH depletion mediated by the bidirectional cystine/Glu antiporter in osteoblastic MC3T3-E1 cells cultured in the absence of differentiation inducers. To further evaluate the possible role of the antiporter in osteoblastogenesis, in this study, we have established stable transfectants of the xCT subunit of the antiporter in MC3T3-E1 cells. Stable overexpression led to a significant facilitation of cellular proliferation determined by different indices with increased GSH levels and decreased ROS generation in addition to promoted [(14)C]cystine incorporation, while Glu failed to significantly inhibit cellular proliferation in stable xCT transfectants. In stable transfectants cultured under differentiation conditions, drastic decreases were invariably seen in Ca(2+) accumulation, alkaline phosphatase activity and several osteoblastic marker gene expressions, in addition to downregulation of mRNA and corresponding protein for runt-related transcription factor-2 (Runx2). Runx2 promoter activity was significantly promoted by the introduction of Runx2 expression vector in a manner sensitive to the prevention by the co-introduction of xCT expression vector in MC3T3-E1 cells. In both MC3T3-E1 cells and murine calvarial osteoblasts cultured with differentiation inducers, transient transfection with xCT siRNA significantly increased Runx2 protein expression along with decreases in xCT mRNA expression and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide reduction. These results suggest that the cystine/Glu antiporter plays a pivotal role in cellular differentiation through a mechanism related to the regulation of transactivation of Runx2 essential for osteoblastogenesis toward maturation in osteoblastic cells.

C/EBPβ binds the P1 promoter of the Runx2 gene and up-regulates Runx2 transcription in osteoblastic cells

The Runx2 factor is an essential component of the regulatory mechanisms that control transcription during skeletogenesis. Runx2/p57 expression in osteoblastic cells is controlled by the P1 promoter, which is recognized by key regulators of osteoblast differentiation including homeodomain factors and Wnt- and BMP-signaling mediators. Here, we report that the transcription factor C/EBPβ up-regulates Runx2/p57 expression by directly binding to the Runx2 P1 promoter in mesenchymal, pre-osteoblastic, and osteoblastic cells. This C/EBPβ-mediated up-regulation is principally dependent on C/EBP site II that is located within the first 180 bp of the proximal P1 promoter region and is highly conserved among mouse, rat, and human Runx2 genes. Our studies reveal how the C/EBPβ factor, known to have a key role during osteogenesis, contributes to regulating the expression of Runx2, the master regulator of osteoblast differentiation.

Gfi1 expressed in bone marrow stromal cells is a novel osteoblast suppressor in patients with multiple myeloma bone disease

Protracted inhibition of osteoblast (OB) differentiation characterizes multiple myeloma (MM) bone disease and persists even when patients are in long-term remission. However, the underlying pathophysiology for this prolonged OB suppression is unknown. Therefore, we developed a mouse MM model in which the bone marrow stromal cells (BMSCs) remained unresponsive to OB differentiation signals after removal of MM cells. We found that BMSCs from both MM-bearing mice and MM patients had increased levels of the transcriptional repressor Gfi1 compared with controls and that Gfi1 was a novel transcriptional repressor of the critical OB transcription factor Runx2. Trichostatin-A blocked the effects of Gfi1, suggesting that it induces epigenetic changes in the Runx2 promoter. MM-BMSC cell-cell contact was not required for MM cells to increase Gfi1 and repress Runx2 levels in MC-4 before OBs or naive primary BMSCs, and Gfi1 induction was blocked by anti-TNF-α and anti-IL-7 antibodies. Importantly, BMSCs isolated from Gfi1(-/-) mice were significantly resistant to MM-induced OB suppression. Strikingly, siRNA knockdown of Gfi1 in BMSCs from MM patients significantly restored expression of Runx2 and OB differentiation markers. Thus, Gfi1 may have an important role in prolonged MM-induced OB suppression and provide a new therapeutic target for MM bone disease.

Osteoblastic γ-aminobutyric acid, type B receptors negatively regulate osteoblastogenesis toward disturbance of osteoclastogenesis mediated by receptor activator of nuclear factor κB ligand in mouse bone

The prevailing view is that signaling machineries for the neurotransmitter GABA are also expressed by cells outside the CNS. In cultured murine calvarial osteoblasts, mRNA was constitutively expressed for both subunits 1 and 2 of metabotropic GABA(B) receptor (GABA(B)R), along with inhibition by the GABA(B)R agonist baclofen of cAMP formation, alkaline phosphatase (ALP) activity, and Ca(2+) accumulation. Moreover, baclofen significantly inhibited the transactivation of receptor activator of nuclear factor-κB ligand (RANKL) gene in a manner sensitive to a GABA(B)R antagonist, in addition to decreasing mRNA expression of bone morphogenetic protein-2 (BMP2), osteocalcin, and osterix. In osteoblastic MC3T3-E1 cells stably transfected with GABA(B)R1 subunit, significant reductions were seen in ALP activity and Ca(2+) accumulation, as well as mRNA expression of osteocalcin, osteopontin, and osterix. In cultured calvarial osteoblasts from GABA(B)R1-null mice exhibiting low bone mineral density in tibia and femur, by contrast, both ALP activity and Ca(2+) accumulation were significantly increased together with promoted expression of both mRNA and proteins for BMP2 and osterix. No significant change was seen in the number of multinucleated cells stained for tartrate-resistant acid phosphatase during the culture of osteoclasts prepared from GABA(B)R1-null mice, whereas a significant increase was seen in the number of tartrate-resistant acid phosphatase-positive multinucleated cells in co-culture of osteoclasts with osteoblasts isolated from GABA(B)R1-null mice. These results suggest that GABA(B)R is predominantly expressed by osteoblasts to negatively regulate osteoblastogenesis through down-regulation of BMP2 expression toward disturbance of osteoclastogenesis after down-regulation of RANKL expression in mouse bone.

Retinoic acid-induced Smad3 expression is required for the induction of osteoblastogenesis of mesenchymal stem cells

Mesenchymal stem cells are pluripotent precursor cells that can differentiate into osteoblasts, adipocytes, chondrocytes and myocytes. Despite their important therapeutic potential little is known about the transcriptional cascades that govern lineage decisions in these cells. Treatment of C3H10T1/2 mouse mesenchymal stem cells with retinoic acid (RA) inhibits adipogenesis and enhances osteoblastogenesis. In particular, RA treatment stimulates the expression of the osteoblast master regulator, runt-related transcription factor 2 (Runx2), whose expression is necessary for the formation of bone. We have shown previously in mesenchymal stem cells that RA acts to stimulate osteoblastogenesis by interfering with the actions of the bzip transcription factor CCAAT/Enhancer Binding Protein beta (C/EBPβ), where it binds to a negative regulatory element within the Runx2 promoter and inhibits its expression. Herein we show that Smad3, whose expression is stimulated by RA, relays the effects of RA on differentiation by initiating the displacement of C/EBPβ from the Runx2 promoter. In addition to stimulating Smad3 expression, RA also stimulated the nuclear localization of this factor, such that in the absence of RA, ectopic Smad3 was unable to drive osteoblastogenesis. While not sufficient to promote osteoblastogenesis, knockdown of Smad3 using a specific shRNA prevented the RA-mediated stimulation of differentiation and displacement of C/EBPβ from the Runx2 P1 promoter. Taken together, these data indicate that Smad3 is an important mediator of RA activity during mesenchymal stem cell differentiation and is necessary for the stimulation of osteoblastogenesis.

A negative correlation between expression profiles of runt-related transcription factor-2 and cystine/glutamate antiporter xCT subunit in ovariectomized mouse bone

We have previously demonstrated that glutamate (Glu) suppresses cellular proliferation toward self-renewal through a mechanism associated with the depletion of intracellular GSH after promoting the retrograde operation of the bidirectional cystine/Glu antiporter in undifferentiated osteoblastic MC3T3-E1 cells. In this study, we investigated the expression profile of the xCT subunit of the antiporter as well as the master regulator of osteoblastogenesis runt-related transcription factor-2 (Runx2) in ovariectomized mouse bone. In spinal columns isolated 28 days after ovariectomy, a marked reduction was seen with the intensity of Von Kossa staining used as an index of ossification. In femurs of these ovariectomized mice, a significant decrease was seen in mRNA and protein levels of Runx2 along with increased expression of both mRNA and the corresponding protein for the xCT subunit. To evaluate the possible role of the antiporter in osteoblastogenesis, stable transfectants were established with the xCT subunit toward the culture with osteoblastic differentiation inducers in MC3T3-E1 cells. In stable xCT transfectants cultured under differentiation conditions, marked decreases were seen in nodule formation, Ca(2+) accumulation, and osteoblastic marker gene expression, in addition to downregulation of both mRNA and the corresponding protein for Runx2. Runx2 promoter activity was markedly stimulated in MC3T3-E1 cells transfected with a responsive promoter plasmid after the culture under differentiation conditions, while transient and stable transfection with xCT expression vector invariably prevented the stimulation through an activator protein-1 site. These results suggest that Runx2 expression would be negatively regulated by the cystine/glutamate antiporter expressed by osteoblastic cells at the level of gene transactivation.

Activating transcription factor 4 regulates osteoclast differentiation in mice

Activating transcription factor 4 (ATF4) is a critical transcription factor for osteoblast (OBL) function and bone formation; however, a direct role in osteoclasts (OCLs) has not been established. Here, we targeted expression of ATF4 to the OCL lineage using the Trap promoter or through deletion of Atf4 in mice. OCL differentiation was drastically decreased in Atf4-/- bone marrow monocyte (BMM) cultures and bones. Coculture of Atf4-/- BMMs with WT OBLs or a high concentration of RANKL failed to restore the OCL differentiation defect. Conversely, Trap-Atf4-tg mice displayed severe osteopenia with dramatically increased osteoclastogenesis and bone resorption. We further showed that ATF4 was an upstream activator of the critical transcription factor Nfatc1 and was critical for RANKL activation of multiple MAPK pathways in OCL progenitors. Furthermore, ATF4 was crucial for M-CSF induction of RANK expression on BMMs, and lack of ATF4 caused a shift in OCL precursors to macrophages. Finally, ATF4 was largely modulated by M-CSF signaling and the PI3K/AKT pathways in BMMs. These results demonstrate that ATF4 plays a direct role in regulating OCL differentiation and suggest that it may be a therapeutic target for treating bone diseases associated with increased OCL activity.

SWI/SNF-independent nuclease hypersensitivity and an increased level of histone acetylation at the P1 promoter accompany active transcription of the bone master gene Runx2

The Runx2 transcription factor is essential for skeletal development as it regulates expression of several key bone-related genes. Multiple lines of evidence indicate that expression of the Runx2/p57 isoform in osteoblasts is controlled by the distal P1 promoter. Alterations of chromatin structure are often associated with transcription and can be mediated by members of the SWI/SNF family of chromatin remodeling complexes, or by transcriptional coactivators that possess enzymatic activities that covalently modify structural components of the chromatin. Here, we report that a specific chromatin remodeling process at the proximal region (residues -400 to 35) of the Runx2 gene P1 promoter accompanies transcriptional activity in osteoblasts. This altered chromatin organization is reflected by the presence of two DNase I hypersensitive sites that span key regulatory elements for Runx2/p57 transcription. Chromatin remodeling and transcription of the Runx2 gene are associated with elevated levels of histone acetylation at the P1 promoter region and binding of active RNA polymerase II and are independent of the activity of the SWI/SNF chromatin remodeling complex. Changes in chromatin organization at the P1 promoter are stimulated during differentiation of C2C12 mesenchymal cells to the osteoblastic lineage by treatment with BMP2. Together, our results support a model in which changes in chromatin organization occur at very early stages of mesenchymal differentiation to facilitate subsequent expression of the Runx2/p57 isoform in osteoblastic cells.

Co-stimulation of the bone-related Runx2 P1 promoter in mesenchymal cells by SP1 and ETS transcription factors at polymorphic purine-rich DNA sequences (Y-repeats)

Transcriptional control of Runx2 gene expression through two alternative promoters (P1 and P2) is critical for the execution of its function as an osteogenic cell fate determining factor. In all vertebrates examined to date, the bone related P1 promoter contains a purine-rich region (-303 to -128 bp in the rat) that separates two regulatory domains. The length of this region differs dramatically between species even within the same order. Using deletion analysis, we show that part of this purine-rich region (-200 to -128) containing a duplicated element (Y-repeat) positively regulates Runx2 P1 transcription. Electrophoretic mobility assays and chromatin immunoprecipitations reveal that Y-repeat binds at least two different classes of transcription factors related to GC box binding proteins (e.g. SP1 and SP7/Osterix) and ETS-like factors (e.g. ETS1 and ELK1). Forced expression of SP1 increases Runx2 P1 promoter activity through the Y-repeats, and small interfering RNA depletion of SP1 decreases Runx2 expression. Similarly, exogenous expression of wild type ELK1, but not a defective mutant that cannot be phosphorylated, enhances Runx2 gene expression. SP1 is most abundant in proliferating cells, and ELK1 is most abundant in postconfluent cells; during MC3T3-E1 osteoblast differentiation, both proteins are transiently co-expressed when Runx2 expression is enhanced. Taken together, our data suggest that basal Runx2 gene transcription is regulated by dynamic interactions between SP1 and ETS-like factors during progression of osteogenesis.

Nuclear factor I transcription factors regulate IGF binding protein 5 gene transcription in human osteoblasts

Insulin-like growth factor binding protein 5 (IGFBP5) is expressed in many cell types including osteoblasts and modulates IGF activities. IGFBP5 may affect osteoblasts and bone formation, in part by mechanisms independent of binding IGFs. The highly conserved IGFBP5 proximal promoter within 100 nucleotides of the start of transcription contains functional cis regulatory elements for C/EBP, Myb and AP-2. We report evidence for a functional Nuclear Factor I (NFI) cis element that mediates activation or repression of IGFBP5 transcription by the NFI gene family. All four NFI genes were expressed in human osteoblast cultures and osteosarcoma cell lines. Co-transfection with human IGFBP5 promoter luciferase reporter and murine Nfi expression vectors showed that Nfib was the most active in stimulating transcription. Nfix was less active and Nfia and Nfic were inhibitory. Knockdown of NFIB and NFIC expression using siRNA decreased and increased IGFBP5 expression, respectively. Analysis of IGFBP5 promoter deletion and mutation reporter constructs identified a functional NFI cis element. All four NFI proteins bound the NFI site in electrophoretic mobility shift experiments and NFIB bound in chromatin immunoprecipitation assays. Results suggest that NFI proteins are important regulators of IGFBP5 expression in human osteoblasts and thus in modulating IGFBP5 functions in bone.

Polycystin-1 regulates skeletogenesis through stimulation of the osteoblast-specific transcription factor RUNX2-II

Polycystin-1 (PC1) may play an important role in skeletogenesis through regulation of the bone-specific transcription factor Runx2-II. In the current study we found that PC1 co-localizes with the calcium channel polycystin-2 (PC2) in primary cilia of MC3T3-E1 osteoblasts. To establish the role of Runx2-II in mediating PC1 effects on bone, we crossed heterozygous Pkd1(m1Bei) and Runx2-II mice to create double heterozygous mice (Pkd1(+/m1Bei)/Runx2-II(+/-)) deficient in both PC1 and Runx2-II. Pkd1(+/m1Bei)/Runx2-II(+/-) mice exhibited additive reductions in Runx2-II expression that was associated with impaired endochondral bone development, defective osteoblast-mediated bone formation, and osteopenia. In addition, we found that basal intracellular calcium levels were reduced in homozygous Pkd1(m1Bei) osteoblasts. In contrast, overexpression of a PC1 C-tail construct increased intracellular calcium and selectively stimulated Runx2-II P1 promoter activity in osteoblasts through a calcium-dependent mechanism. Site-directed mutagenesis of critical amino acids in the coiled-coil domain of PC1 required for coupling to PC2 abolished PC1-mediated Runx2-II P1 promoter activity. Additional promoter analysis mapped the PC1-responsive region to the "osteoblast-specific" enhancer element between -420 and -350 bp that contains NFI and AP-1 binding sites. Chromatin immunoprecipitation assays confirmed the calcium-dependent binding of NFI to this region. These findings indicate that PC1 regulates osteoblast function through intracellular calcium-dependent control of Runx2-II expression. The overall function of the primary cilium-polycystin complex may be to sense and transduce environmental clues into signals regulating osteoblast differentiation and bone development.

cDNA cloning of Runx family genes from the pufferfish (Fugu rubripes)

The Runx family genes are involved in hematopoiesis, osteogenesis and neuropoiesis, and mutations in these genes have been frequently associated with human hereditary diseases and cancers. Here we report the cDNA cloning of the full Runx gene family of the pufferfish (Fugu rubripes), which comprises frRunx1, frRunx2, frRunx3, frRunt and frCbfb. Fugu is evolutionarily distant from mammals, thus the annotation of the frRunx family genes greatly facilitates comparative genomics approaches. Protein sequence comparison revealed that the fugu genes show high conservation in the Runt domain and PY and VWRPY motifs. frRunx1 had an extra stretch of eight histidine residues, while frRunx2 lacked the poly-glutamine/-alanine stretch that is a hallmark of the mammalian Runx2 genes. Analysis of the promoter regions revealed high conservation of the binding sites for transcription factors, including Runx sites in the P1 promoters. Abundant CpG dinucleotides in the P2 promoter regions were also detected. The expression patterns of the frRunx family genes in various tissues showed high similarity to those of the mammalian Runx genes. The genomic structures of the fugu and mammalian Runx genes are largely conserved except for a split exon 2 in frRunx1 and an extra exon in the C-terminal region of frRunx3 that is missing in mammalian Runx3 genes. The similarities and differences between the Runx family genes of fugu and mammals will improve our understanding of the functions of these proteins.

Elevated circulating level of osteopontin is associated with advanced disease state of non-small cell lung cancer

Osteopontin (OPN) plays important roles in tumor progression and metastasis through binding to OPN receptors such as alpha(v)beta(beta) integrin and CD44, and its overexpression in tumor is associated poor clinical outcome of NSCLC patients. Circulating OPN levels, measured by ELISA in 130 NSCLC cases that had not been treated for cancer at the time of sampling, were analyzed according to clinical, pathologic parameters and single nucleotide polymorphisms (SNPs) in the OPN gene promoter. Advanced disease states had higher circulating levels of OPN (T4 versus T1-3, N3 versus N0-2, and M1 versus M0, P=.029, .001, and .001, respectively, Kruskal-Wallis H-test), reflected by higher level of OPN in stage IV than stage I-III (P=.029, Kruskal-Wallis H-test). Among the clinical and pathological parameters including age, gender, smoking status, histologic subtypes and grade of differentiation, smoking status influences circulating OPN level showing higher level of OPN in ex-smokers than current and non-smokers (P=.038, Kruskal-Wallis H-test). Variation at nucleotide (nt) -443 of the OPN gene promoter had no influence on circulating OPN levels, however, patients with G/G at nt -156 showed higher concentrations of OPN than those with G/GG or GG/GG (P=.003, Kruskal-Wallis H-test). A patient with G/G at nt -156 was more frequently diagnosed with advanced stage (IIIB-IV) than with early stage (I-IIIA) NSCLC (P=.048, Mantel-Haenszel-test). In multivariate analysis, stage is the only independent factor influencing circulating level of OPN. Although circulating level of OPN in the patients with bone metastasis was higher than in those without bone metastasis (P=.028, Mann-Whitney U-test), there was no difference in the OPN levels between bone metastasis group and non-bone metastasis group. Given that the elevated levels of OPN is associated with advanced stages of NSCLC, elucidating OPN regulatory mechanisms may contribute to the development of a new therapeutic modality for NSCLC.

Regulatory roles of Runx2 in metastatic tumor and cancer cell interactions with bone

The three mammalian Runt homology domain transcription factors (Runx1, Runx2, Runx3) support biological control by functioning as master regulatory genes for the differentiation of distinct tissues. Runx proteins also function as cell context-dependent tumor suppressors or oncogenes. Abnormalities in Runx mediated gene expression are linked to cell transformation and tumor progression. Runx2 is expressed in mesenchymal linage cells committed to the osteoblast phenotype and is essential for bone formation. This skeletal transcription factor is aberrantly expressed at high levels in breast and prostate tumors and cells that aggressively metastasize to the bone environment. In cancer cells, Runx2 activates expression of bone matrix and adhesion proteins, matrix metalloproteinases and angiogenic factors that have long been associated with metastasis. In addition, Runx2 mediates the responses of cells to signaling pathways hyperactive in tumors, including BMP/TGFbeta and other growth factor signals. Runx2 forms co-regulatory complexes with Smads and other co-activator and co-repressor proteins that are organized in subnuclear domains to regulate gene transcription. These activities of Runx2 contribute to tumor growth in bone and the accompanying osteolytic disease, established by interfering with Runx2 functions in metastatic breast cancer cells. Inhibition of Runx2 in MDA-MB-231 cells transplanted to bone decreased tumorigenesis and prevented osteolysis. This review evaluates evidence that Runx2 regulates early metastatic events in breast and prostate cancers, tumor growth, and osteolytic bone disease. Consideration is given to the potential for inhibition of this transcription factor as a therapeutic strategy upstream of the regulatory events contributing to the complexity of metastasis to bone.

HOXA10 controls osteoblastogenesis by directly activating bone regulatory and phenotypic genes

HOXA10 is necessary for embryonic patterning of skeletal elements, but its function in bone formation beyond this early developmental stage is unknown. Here we show that HOXA10 contributes to osteogenic lineage determination through activation of Runx2 and directly regulates osteoblastic phenotypic genes. In response to bone morphogenic protein BMP2, Hoxa10 is rapidly induced and functions to activate the Runx2 transcription factor essential for bone formation. A functional element with the Hox core motif was characterized for the bone-related Runx2 P1 promoter. HOXA10 also activates other osteogenic genes, including the alkaline phosphatase, osteocalcin, and bone sialoprotein genes, and temporally associates with these target gene promoters during stages of osteoblast differentiation prior to the recruitment of RUNX2. Exogenous expression and small interfering RNA knockdown studies establish that HOXA10 mediates chromatin hyperacetylation and trimethyl histone K4 (H3K4) methylation of these genes, correlating to active transcription. HOXA10 therefore contributes to early expression of osteogenic genes through chromatin remodeling. Importantly, HOXA10 can induce osteoblast genes in Runx2 null cells, providing evidence for a direct role in mediating osteoblast differentiation independent of RUNX2. We propose that HOXA10 activates RUNX2 in mesenchymal cells, contributing to the onset of osteogenesis, and that HOXA10 subsequently supports bone formation by direct regulation of osteoblast phenotypic genes.

BMP2 commitment to the osteogenic lineage involves activation of Runx2 by DLX3 and a homeodomain transcriptional network

Several homeodomain (HD) proteins are critical for skeletal patterning and respond directly to BMP2 as an early step in bone formation. RUNX2, the earliest transcription factor proven essential for commitment to osteoblastogenesis, is also expressed in response to BMP2. However, there is a gap in our knowledge of the regulatory cascade from BMP2 signaling to the onset of osteogenesis. Here we show that BMP2 induces DLX3, a homeodomain protein that activates Runx2 gene transcription. Small interfering RNA knockdown studies in osteoblasts validate that DLX3 is a potent regulator of Runx2. Furthermore in Runx2 null cells, DLX3 forced expression suffices to induce transcription of Runx2, osteocalcin, and alkaline phosphatase genes, thus defining DLX3 as an osteogenic regulator independent of RUNX2. Our studies further show regulation of the Runx2 gene by several homeodomain proteins: MSX2 and CDP/cut repress whereas DLX3 and DLX5 activate endogenous Runx2 expression and promoter activity in non-osseous cells and osteoblasts. These HD proteins exhibit distinct temporal expression profiles during osteoblast differentiation as well as selective association with Runx2 chromatin that is related to Runx2 transcriptional activity and recruitment of RNA polymerase II. Runx2 promoter mutagenesis shows that multiple HD elements control expression of Runx2 in relation to the stages of osteoblast maturation. Our studies establish mechanisms for commitment to the osteogenic lineage directly through BMP2 induction of HD proteins DLX3 and DLX5 that activate Runx2, thus delineating a transcriptional regulatory pathway mediating osteoblast differentiation. We propose that the three homeodomain proteins MSX2, DLX3, and DLX5 provide a key series of molecular switches that regulate expression of Runx2 throughout bone formation.

Identification of USF2 as a key regulator of Runx2 expression in mouse pluripotent mesenchymal D1 cells

Runx2 is one of the most important transcription factors directing the osteogenesis of mesenchymal stem cells and osteoblastic functions. It is likely that the factors controlling Runx2 expression would trigger the early steps of osteoblast differentiation. By using a reporter gene assay for 4.5 kb Runx2 promoter, it was found that the first 305 bp of Runx2 promoter are active in D1 cells. Within this region, electromobility shift assays (EMSAs) delineated a 6 bp of CACATG bound specifically by the proteins from D1 cell nuclear extract. Antibody super-shift and DNA-coupling magnetic bead pull-down assay indicated that the protein bound to this sequence is USF2. Site-specific mutagenesis revealed that this sequence contributed to the activity of 305 bp Runx2 promoter. Thus, we suggest that USF2 might be one of the regulators for the expression of the Runx2 gene in D1 cells.

Genome-wide analysis of p53 under hypoxic conditions

Hypoxia is an important nongenotoxic stress that modulates the tumor suppressor activity of p53 during malignant progression. In this study, we investigated how genotoxic and nongenotoxic stresses regulate p53 association with chromatin, p53 transcriptional activity, and p53-dependent apoptosis. We found that genotoxic and nongenotoxic stresses result in the accumulation and binding of the p53 tumor suppressor protein to the same cognate binding sites in chromatin. However, it is the stress that determines whether downstream signaling is mediated by association with transcriptional coactivators. In contrast to p53 induced by DNA-damaging agents, hypoxia-induced p53 has primarily transrepression activity. Using extensive microarray analysis, we identified families of repressed targets of p53 that are involved in cell signaling, DNA repair, cell cycle control, and differentiation. Following our previous study on the contribution of residues 25 and 26 to p53-dependent hypoxia-induced apoptosis, we found that residues 25-26 and 53-54 and the polyproline- and DNA-binding regions are also required for both gene repression and the induction of apoptosis by p53 during hypoxia. This study defines a new role for residues 53 and 54 of p53 in regulating transrepression and demonstrates that 25-26 and 53-54 work in the same pathway to induce apoptosis through gene repression.

Canonical WNT signaling promotes osteogenesis by directly stimulating Runx2 gene expression

Both activating and null mutations of proteins required for canonical WNT signaling have revealed the importance of this pathway for normal skeletal development. However, tissue-specific transcriptional mechanisms through which WNT signaling promotes the differentiation of bone-forming cells have yet to be identified. Here, we address the hypothesis that canonical WNT signaling and the bone-related transcription factor RUNX2/CBFA1/AML3 are functionally linked components of a pathway required for the onset of osteoblast differentiation. Our findings show that, in bone of the SFRP1 (secreted frizzled-related protein-1)-null mouse, which exhibits activated WNT signaling and a high bone mass phenotype, there is a significant increase in expression of T-cell factor (TCF)-1, Runx2, and the RUNX2 target gene osteocalcin. We demonstrate by mutational analysis that a functional TCF regulatory element responsive to canonical WNT signaling resides in the promoter of the Runx2 gene (-97 to -93). By chromatin immunoprecipitation, recruitment of beta-catenin and TCF1 to the endogenous Runx2 gene is shown. Coexpression of TCF1 with canonical WNT proteins resulted in a 2-5-fold activation of Runx2 promoter activity and a 7-8-fold induction of endogenous mRNA in mouse pluripotent mesenchymal and osteoprogenitor cells. This enhancement was abrogated by SFRP1. Taken together, our results provide evidence for direct regulation of Runx2 by canonical WNT signaling and suggest that Runx2 is a target of beta-catenin/TCF1 for the stimulation of bone formation. We propose that WNT/TCF1 signaling, like bone morphogenetic protein/transforming growth factor-beta signaling, activates Runx2 gene expression in mesenchymal cells for the control of osteoblast differentiation and skeletal development.

The balancing act of transcription factors C-1-1 and Runx2 in articular cartilage development

In previous studies we found that the ets transcription factor C-1-1 is involved in articular chondrocyte development, and we and others found that the transcription factor Runx2 is required for growth plate chondrocyte maturation and ossification. We determined here whether the two factors exert reciprocal influences on their expression and function and in so doing, steer chondrocyte developmental paths. Virally driven Runx2 over-expression in cultured chick chondrocytes did indeed lead to decreased C-1-1 expression, accompanied by decreased expression of articular cartilage marker tenascin-C, decreased proliferation, and increased expression of maturation marker collagen X. In good agreement, over-expression of a dominant-negative Runx2 form had opposite phenotypic consequences. When C-1-1 itself was over-expressed in chondrocytes already undergoing maturation, maturation was halted and the cells became small, rich in tenascin-C, and mitotically quite active. To extend these observations, we misexpressed C-1-1 in mouse cartilage and found that it caused a severe inhibition of chondrocyte maturation and widespread tenascin-C expression. In sum, C-1-1 and Runx2 do influence their respective expression patterns. The factors are powerful chondrocyte regulators and their functional interrelationships may be important for steering the cells toward alternative developmental paths.

The bone-specific expression of Runx2 oscillates during the cell cycle to support a G1-related antiproliferative function in osteoblasts

The Runx2 (CBFA1/AML3/PEBP2alphaA) transcription factor promotes skeletal cell differentiation, but it also has a novel cell growth regulatory activity in osteoblasts. We addressed here whether Runx2 activity is functionally linked to cell cycle-related mechanisms that control normal osteoblast proliferation and differentiation. We found that the levels of Runx2 gene transcription, mRNA and protein, are each up-regulated with cessation of cell growth (i.e. G(0)/G(1) transition) in preconfluent MC3T3 osteoblastic cells that do not yet express mature bone phenotypic gene expression. Cell growth regulation of Runx2 is also observed in primary calvarial osteoblasts and other osteoblastic cells with relatively normal cell growth characteristics, but not in osteosarcoma cells (e.g. SAOS-2 and ROS17/2.8). Runx2 levels are cell cycle-regulated in MC3T3 cells with respect to the G(1)/S and M/G(1) transitions: oscillates from maximal expression levels during early G(1) to minimal levels during early S phase and mitosis. However, in normal or immortalized (e.g. ATDC5) chondrocytic cells, Runx2 expression is suppressed during quiescence, and Runx2 levels are not regulated during G(1) and S phase in ATDC5 cells. Antisense or small interfering RNA-mediated reduction of the low physiological levels of Runx2 in proliferating MC3T3 cells does not accelerate cell cycle progression. However, forced expression of Runx2 suppresses proliferation of MC3T3 preosteoblasts or C2C12 mesenchymal cells which have osteogenic potential. Forced elevation of Runx2 in synchronized MC3T3 cells causes a delay in G(1). We propose that Runx2 levels and function are biologically linked to a cell growth-related G(1) transition in osteoblastic cells.

Control of RUNX2 isoform expression: The role of promoters and enhancers

The three mammalian RUNX genes constitute the family of runt domain transcription factors that are involved in the regulation of a number of developmental processes such as haematopoiesis, osteogenesis and neuronal differentiation. All three genes show a complex temporo-spatial pattern of expression. Since the three proteins are probably mutually interchangeable with regard to function, most of the specificity of each family member seems to be based on a tightly controlled regulation of expression. While RUNX gene expression is driven by two promoters for each gene, the promoter sequence alone does not seem to suffice for a proper expressional control. This review focuses on the available evidence for the existence of such control mechanisms and studies aiming at discovering cis-acting regulatory sequences of the RUNX2 gene.

Effect of β-alanyl-L-histidinato zinc on the differentiation of C2C12 cells

Although beta-alanyl-L-histidinato zinc (AHZ) can promote osteoblast differentiation, the molecular mechanism responsible is not fully understood. The purpose of this study was to determine the effect of AHZ on undifferentiating mesenchymal cells. C2C12, a typical pluripotential mesenchymal cell line, was used. The cells were cultured in 5% serum-containing medium to induce differentiation, either with or without the addition of AHZ. Cell lineage was determined by immunostaining of type II myosin heavy chains, alkaline phosphatase (ALPase) activity, mRNA expression of cellular phenotype-specific markers using semi-quantitative reverse transcriptase-polymerase chain reaction, and core binding factor alpha1/runt-related transcription factor-2 (Cbfa1/Runx2) protein synthesis using Western blot analysis. C2C12 cells cultured in the presence of AHZ were strongly inhibited from developing into myoblasts, and showed high ALPase activity that was approximately double that in the vehicle. The expression of mRNA for Cbfa1/Runx2, ALPase, Sox9 and type X collagen was increased markedly by the AHZ-stimulated medium, whereas that of desmin and MyoD mRNA was drastically decreased. AHZ increased Cbfa1/Runx2 protein expression substantially. These results provide clear evidence that AHZ converts the differentiation pathway of C2C12 cells to the osteoblast and/or chondroblast lineage.

Different effects of BMP-2 on marrow stromal cells from human and rat bone

Bone morphogenetic proteins (BMPs) promote the differentiation of osteoprogenitor cells, and also induce osteogenesis in bone marrow stromal cells (MSC) from rats and mice. However, compared to results with animal models, BMPs are relatively inefficient in inducing human MSC to undergo osteogenesis, and are much less effective in promoting bone formation in human clinical trials. Previous studies indicated that, while human MSC respond to dexamethasone with elevated levels of the osteoblast marker alkaline phosphatase, most isolates of human MSC fail to show alkaline phosphatase induction in response to BMP-2, BMP-4, or BMP-7. Several other genes known to be induced by BMPs are appropriately regulated; thus, human MSC are capable of some BMP-activated signaling. Analysis of the BMP receptors ALK-3 and ALK-6 indicated that, although ALK-6 mRNA was not expressed in human MSC, overexpressing a constitutively active ALK-6 receptor did not induce elevated alkaline phosphatase. Real-time RT-PCR was used to investigate expression of several osteoblast-related transcription factors in MSC after 6 days' exposure to BMP2 or dexamethasone. Msx-2, a transcription factor that has been reported to inhibit differentiation of osteoprogenitor cells, showed 10-fold elevation in BMP-2-treated human MSC, but not in BMP-2-treated rat MSC. Overexpression of Msx-2 in human and rat MSC, however, did not alter alkaline phosphatase levels, which suggests that absence of BMP-stimulated alkaline phosphatase was not caused by the BMP-2-induced increase in Msx-2. Although Runx2 isoforms have been implicated in control of osteoblast differentiation, levels of this transcription factor were unaffected by BMP treatment. Expression of the FKHR transcription factor, which has been reported to regulate alkaline phosphatase transcription in mouse cells, showed a modest increase in response to BMP-2, but a much greater increase in dexamethasone-treated cells. We propose that BMP regulation of the bone/liver/kidney alkaline phosphatase gene is indirect, requiring expression of new transcription factor(s) that behave differently in rodent and human MSC.

Structure and regulated expression of mammalian RUNX genes

The RUNX are key regulators of lineage-specific gene expression in major developmental pathways. The expression of RUNX genes is tightly regulated, leading to a highly specific spatio/temporal expression pattern and to distinct phenotypes of gene knockouts. This review highlights the extensive structural similarities between the three mammalian RUNX genes and delineates how regulation of their expression at the levels of transcription and translation are orchestrated into the unique RUNX expression pattern.

Effect of β-alanyl-L-histidinato zinc on the differentiation pathway of human periodontal ligament cells

We examined the effect of beta-alanyl-L-histidinato zinc (AHZ) on the differentiation of human periodontal ligament (HPDL) cells. HPDL cells were cultured with alpha-minimum essential medium containing 10% fetal bovine serum with or without 10(-4) or 10(-5) M AHZ for up to 10 days. The expression of runt-related transcription factor-2/core binding factor alpha-1 (RUNX2/Cbfa1), Sox9, bone morphogenetic proteins (BMPs), and BMP receptors was measured using semiquantitative reverse transcription-polymerase chain reactions. Phosphorylation of Smad1 was determined using a Western blot analysis. RUNX2/Cbfa1 expression increased markedly in cells cultured with AHZ, while Sox9 expression increased slightly. BMP-7 expression was much higher than that of controls in cultures with AHZ, whereas BMP-2 expression was only slightly higher. The expression of BMP receptors increased markedly in cells cultured with AHZ. The phosphorylation of Smad1, a signal-transducing molecule for BMP-2 and BMP-7, was increased markedly in cultures with AHZ. The results suggest that AHZ diverts the differentiation pathway of HPDL cells to the osteoblast lineage via BMP-2 or BMP-7.

Identification of potential modifiers of Runx2/Cbfa1 activity in C2C12 cells in response to bone morphogenetic protein-7

Treatment with BMP-7 causes a shift in the differentiation pathway from myoblastic to osteoblastic in C2C12 mouse myoblast precursor cells in vitro. The underlying molecular mechanism is largely unknown. BMP-7 at 200 ng/ml completely inhibited myotube formation in C2C12 cells and dramatically induced alkaline phosphatase activity up to 20-fold when compared to untreated cells by day 12 in culture. The level of Runx2/Cbfa1 mRNA, a bone-specific transcription factor, was also stimulated up to 6-fold by BMP-7 with a peak at 24 h. In addition BMP-7 treatment stimulated a 55-fold increase in osteocalcin mRNA as early as 24 h after treatment. A novel finding was that the expression of the chondrocyte markers Sox9 and type II collagen was increased as well. Runx2/Cbfa1 is a molecular switch for osteoblast differentiation. To initiate the study of modulators of Runx2/Cbfa1, such as kinases and cofactors, during osteoblastic differentiation of C2C12 cells treated by BMP-7 in vitro, microarray analyses of gene expressions were performed. Microarray data suggested that a total of 882 transcripts were either up- or downregulated at least 2-fold. Cluster analyses revealed 76 genes (including ESTs) with expression patterns that paralleled Runx2/Cbfa1. Thirteen of these 76 genes were initially selected as potential transcription modulators for further study; including CCAAT/enhancer binding protein delta, distal- less homeobox 1, forkhead box F2, insulin-like growth factor binding protein 4, an ortholog of human osteoclast stimulating factor 1 and p300/CBP-associated factor. Some transcription modulators have been associated with osteoblastic differentiation or interacted with Runx2/Cbfa1. Most of them have not been extensively studied in osteoblastic differentiation and in relationship to Runx2/Cbfa1. Thus, these studies identify potential regulators for Runx2/Cbfa1 and osteoblast differentiation. In addition, our data revealed for the first time that BMP-7 not only induced the expression of osteoblastic differentiation markers but also stimulated the expression of chondroblastic markers in C2C12 cells.