MAPK pathways activate and phosphorylate the osteoblast-specific transcription factor, Cbfa1

Phosphophoryn regulates the gene expression and differentiation of NIH3T3, MC3T3-E1, and human mesenchymal stem cells via the integrin/MAPK signaling pathway

Extracellular matrix proteins (ECMs) serve as both a structural support for cells and a dynamic biochemical network that directs cellular activities. ECM proteins such as those of the SIBLING family (small integrin-binding ligand glycoprotein) could possess inherent growth factor activity. In this study, we demonstrate that exon 5 of dentin matrix protein 3 (phosphophoryn (PP)), a non-collagenous dentin ECM protein and SIBLING protein family member, up-regulates osteoblast marker genes in primary human adult mesenchymal stem cells (hMSCs), a mouse osteoblastic cell line (MC3T3-E1), and a mouse fibroblastic cell line (NIH3T3). Quantitative real-time PCR technology was used to quantify gene expression levels of bone markers such as Runx2, Osx (Osterix), bone/liver/kidney Alp (alkaline phosphatase), Ocn (osteocalcin), and Bsp (bone sialoprotein) in response to recombinant PP and stably transfected PP. PP up-regulated Runx2, Osx, and Ocn gene expression. PP increased OCN protein production in hMSCs and MC3T3-E1. ALP activity and calcium deposition was increased by PP in hMSC. Furthermore, an alpha(v)beta(3) integrin-blocking antibody significantly inhibited recombinant PP-induced expression of Runx2 in hMSCs, suggesting that signaling by PP is mediated through the integrin pathway. PP was also shown to activate p38, ERK1/2, and JNK, three components of the MAPK pathway. These data demonstrate a novel signaling function for PP in cell differentiation beyond the hypothesized role of PP in biomineralization.

Bisphosphonates and estrogens inhibit osteocyte apoptosis via distinct molecular mechanisms downstream of extracellular signal-regulated kinase activation

Both estrogens and bisphosphonates attenuate osteocyte apoptosis by activating the extracellular signal-regulated kinases (ERKs). However, whereas estrogens activate ERKs via an extranuclear function of the estrogen receptor, bisphosphonates do so by opening connexin 43 hemichannels. Here, we demonstrated that the signaling events downstream of ERKs induced by these two stimuli are also distinct. Inhibition of osteocyte apoptosis by estrogens requires nuclear accumulation of ERKs and activation of downstream transcription factors. On the other hand, anti-apoptosis induced by bisphosphonates requires neither transcription nor ERK-dependent transcription factors. Instead, the effect of bisphosphonates is abolished when ERKs are restricted to the nucleus by blocking CRM1/exportin1-mediated nuclear protein export or by expressing nuclear-anchored ERKs, but it is unaffected in cells expressing cytoplasmic-anchored ERKs. Connexin 43/ERK-mediated anti-apoptosis induced by bisphosphonates requires the kinase activity of the cytoplasmic target of ERKs, p90(RSK), which in turn phosphorylates the pro-apoptotic protein BAD and C/EBPbeta. Phosphorylation of BAD renders it inactive, whereas phosphorylation of C/EBPbeta leads to binding of pro-caspases, thus inhibiting apoptosis independently of the transcriptional activity of this transcription factor. Consistent with the evidence that estrogens and bisphosphonates phosphorylate diverse targets of ERKs, probably resulting from activation of spatially distinct pools of these kinases, the two agents had additive effects on osteocyte survival.

Growth hormone attenuates the transcriptional activity of Runx2 by facilitating its physical association with Stat3beta

We document that GH controls osteoblast function by modulating the biological activity of the osteospecific transcription factor Runx2. Evidence is provided for a physical interaction between Runx2 and Stat3beta, which is enhanced by GH and downregulates the transcriptional properties of this key osteogenic regulator.

INTRODUCTION

Growth hormone (GH) signals to bone either through insulin-like growth factor-1 or directly by influencing the function of osteoblasts, the bone-forming cells. This study aimed at exploring the molecular events that underlie the direct biological action of GH on osteoblastic cells, and specifically, the effects that it might exert on the function of the bone-specific transcriptional regulator Runx2.

MATERIALS AND METHODS

The GH-responsive human osteoblastic cell line Saos-2 was used as our experimental system. Western blot analyses were used to monitor the presence of several parameters known to be affected by GH in these cells (i.e., downregulation of GH receptor, induction of STATs, and extracellular signal-regulated kinase [ERK] mitogen-activated protein kinase [MAPK] pathways). Electrophoretic mobility shift assays were used to assess Runx2 and Stat3 binding activity on an osteoblast-specific element (OSE2) after GH treatment. A combination of yeast two-hybrid and co-immunoprecipitation assays were performed to test for the existence of a physical Runx2.Stat3beta association. Finally, co-transfection experiments were used to investigate the interplay of the two transcription factors on the activity of a p6OSE2-Luc promoter after GH stimulation.

RESULTS

We show that GH signaling through Stat3/ERK MAPK potentiates the DNA binding activity of Runx2 but, at the same time, restrains its transcriptional potential. Moreover, a novel physical interaction of Runx2 with transcription factor Stat3beta, which is enhanced by GH stimulation, was documented both in vitro and in vivo. Importantly, this interaction impairs the transcriptional activity of Runx2 without affecting its DNA binding capacity.

CONCLUSION

Our data provide the first evidence that GH modulates the transcriptional function of Runx2 in osteoblastic cells by promoting its inhibitory interaction with Stat3beta. Shedding light on such mechanisms will contribute to a better understanding of GH effects on skeletal homeostasis that may impact on decisions at the clinical level, especially in diseases affecting bone quantity and quality (e.g., osteoporosis).

Runx2/Cbfa1 stimulates transdifferentiation of primary skeletal myoblasts into a mineralizing osteoblastic phenotype

Runx2, a transcriptional activator downstream of bone morphogenetic protein (BMP) signaling, is essential to osteoblastic differentiation and bone formation and maintenance. BMPs activate complex signaling networks, utilizing numerous signaling molecules and transcription factors to induce expression of osteoblastic markers in mesenchymal cell types. However, the role of Runx2 in this process, particularly in an environment independent of the other regulatory elements modulated by BMPs, remains poorly understood. In the present study, we used retroviral gene delivery to examine the effects of sustained Runx2 expression in primary myoblasts. Runx2 inhibited myogenesis, as demonstrated by suppression of MyoD and myogenin mRNA levels and reduced myotube formation. Additionally, Runx2-stimulated osteogenesis including osteoblastic gene expression, alkaline phosphatase activity, and biological mineral deposition. Notably, these osteogenic markers were induced to significantly greater levels than those observed in BMP-2-treated controls. These results demonstrate that direct exogenous expression of the Runx2 transcription factor, only one of numerous downstream targets of BMP signaling, is sufficient to induce transdifferentiation of myogenic cells into a mineralizing osteogenic lineage. This work underscores the potency of Runx2 as a regulator of osteogenesis and cell differentiation and provides new insights into the plasticity of committed mesenchymal cells.

Modeled microgravity disrupts collagen I/integrin signaling during osteoblastic differentiation of human mesenchymal stem cells

Spaceflight leads to reduced bone mineral density in weight bearing bones that is primarily attributed to a reduction in bone formation. We have previously demonstrated severely reduced osteoblastogenesis of human mesenchymal stem cells (hMSC) following 7 days culture in modeled microgravity (MMG). One potential mechanism for reduced osteoblastic differentiation is disruption of type I collagen (Col I)-integrin interactions and reduced integrin signaling. Integrins are heterodimeric transmembrane receptors that bind extracellular matrix (ECM) proteins and produce signals essential for proper cellular function, survival, and differentiation. Therefore, we investigated the effects of MMG on integrin expression and function in hMSC. We demonstrate that 7 days of culture in MMG leads to reduced expression of the ECM protein, Col I. Conversely, MMG consistently increases Col I-specific alpha2 and beta1 integrin protein expression. Despite this increase in integrin subunit expression, autophosphorylation of adhesion-dependent kinases, focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (PYK2), is significantly reduced. Activation of Akt protein kinase (Akt) is unaffected by the reduction in FAK activation. However, reduced downstream signaling via the Ras-mitogen activated protein kinase (MAPK) pathway is evidenced by a reduction in Ras and extracellular signal-related protein kinase (ERK) activation. Taken together, our findings indicate that MMG decreases integrin/MAPK signaling, which likely contributes to the observed reduction in osteoblastogenesis.

Insulin-like Growth Factor-1 Regulates Endogenous RUNX2 Activity in Endothelial Cells through a Phosphatidylinositol 3-Kinase/ERK-dependent and Akt-independent Signaling Pathway

Insulin-like growth factor-1 (IGF-1) is an angiogenic and oncogenic factor that activates signal transduction pathways involved in the expression of transcriptional regulators of tumorigenesis. RUNX2, a member of the Ig-loop family of transcription factors is expressed in vascular endothelial cells (EC) and regulates EC migration, invasion, and proliferation. Here we show that IGF-1 and its receptor regulate post-translational changes in RUNX2 to activate DNA binding in proliferating EC. The phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002, reduced both basal and IGF-1-stimulated RUNX2 DNA binding activity in the absence of changes in RUNX2 protein as did the overexpression of the phosphatidylinositol 3-phosphate phosphatase, confirming that PI3K signaling mediates RUNX2 activation. IGF-1 increased ERK1/2 activation, which was abrogated by the inhibition of PI3K, thus linking these two pathways in EC. Treatment with U0126, which inhibits ERK1/2 activation, reduced IGF-1-stimulated RUNX2 DNA binding without affecting RUNX2 protein levels. Overexpression of constitutively active MKK1 increased RUNX2 DNA binding and phosphorylation. No additive effects of PI3K or ERK inhibitors on DNA binding were evident. Surprisingly, these IGF-1-mediated effects on RUNX2 were not regulated by Akt phosphorylation, a common downstream target of PI3K, as determined by pharmacological or genetic inhibition. However, an inhibitor of the p21-activated protein kinase-1, glutathione S-transferase-Pak1-(83-149), inhibited both basal and IGF-1-stimulated RUNX2 DNA binding, suggesting that Pak1 mediates IGF-1 signaling to increase RUNX2 activity. These results indicate that the angiogenic growth factor, IGF-1, can regulate RUNX2 DNA binding through sequential activation of the PI3K/Pak1 and ERK1/2 signaling cascade.

A soluble form of fibroblast growth factor receptor 2 (FGFR2) with S252W mutation acts as an efficient inhibitor for the enhanced osteoblastic differentiation caused by FGFR2 activation in Apert syndrome

Apert syndrome is an autosomal dominant disease characterized by craniosynostosis and bony syndactyly associated with point mutations (S252W and P253R) in the fibroblast growth factor receptor (FGFR) 2 that cause FGFR2 activation. Here we investigated the role of the S252W mutation of FGFR2 on osteoblastic differentiation. Osteoblastic cells derived from digital bone in two Apert patients with the S252W mutation showed more prominent alkaline phosphatase activity, osteocalcin and osteopontin mRNA expression, and mineralized nodule formation compared with the control osteoblastic cells derived from two independent non-syndromic polydactyly patients. Stable clones of the human MG63 osteosarcoma cells (MG63-Ap and MG63-IIIc) overexpressing a splice variant form of FGFR2 with or without the S252W mutation (FGFR2IIIcS252W and FGFR2IIIc) showed a higher RUNX2 mRNA expression than parental MG63 cells. Furthermore MG63-Ap exhibited a higher osteopontin mRNA expression than did MG63-IIIc. The enhanced osteoblastic marker gene expression and mineralized nodule formation of the MG63-Ap was inhibited by the conditioned medium from the COS-1 cells overexpressing the soluble FGFR2IIIcS252W. Furthermore the FGF2-induced osteogenic response in the mouse calvarial organ culture system was blocked by the soluble FGFR2IIIcS252W. These results show that the S252W mutation in the FGFR2 gene enhances the osteoblast phenotype in human osteoblasts and that a soluble FGFR2 with the S252W mutation controls osteoblast differentiation induced by the S252W mutation through a dominant negative effect on FGFR2 signaling in Apert syndrome.

Regulation of Notch signaling genes during BMP2-induced differentiation of osteoblast precursor cells

The bone morphogenetic protein (BMP)-induced Smad signal transduction pathway is an important positive regulator of osteoblast differentiation. BMP and other members of the transforming growth factor-beta (TGF-beta) family have distinct effects on osteoblast differentiation, depending on cell type and cell differentiation status. In C2C12 mesenchymal cells, BMP-induced osteoblast differentiation can be blocked by TGF-beta. In a search for key regulators of osteoblast differentiation we have used microarray analysis to identify genes which are differentially regulated by BMP2 and TGF-beta. Within the first 24 h following the onset of differentiation, 61 BMP2-regulated genes were identified of which the BMP2 effect was counteracted by TGF-beta. The majority of these differentially expressed transcripts are related to signal transduction. Notably, our data show that three Notch signal transduction pathway genes, Lfng, Hey1, and Hes1, are differentially regulated by BMP2 and TGF-beta. This suggests that these genes might function as the focal point for interaction of Smad and Notch signaling during osteoblast differentiation.

Histone deacetylase 3 interacts with runx2 to repress the osteocalcin promoter and regulate osteoblast differentiation

The Runt domain transcription factor Runx2 (AML-3, and Cbfa1) is essential for osteoblast development, differentiation, and bone formation. Runx2 positively or negatively regulates osteoblast gene expression by interacting with a variety of transcription cofactor complexes. In this study, we identified a trichostatin A-sensitive autonomous repression domain in the amino terminus of Runx2. Using a candidate approach, we found that histone deacetylase (HDAC) 3 interacts with the amino terminus of Runx2. In transient transfection assays, HDAC3 repressed Runx2-mediated activation of the osteocalcin promoter. HDAC inhibitors and HDAC3-specific short hairpin RNAs reversed this repression. In vivo, Runx2 and HDAC3 associated with the osteocalcin promoter. These data indicate that HDAC3 regulates Runx2-mediated transcription of osteoblast genes. Suppression of HDAC3 in MC3T3 preosteoblasts by RNA interference accelerated the expression of Runx2 target genes, osteocalcin, osteopontin, and bone sialoprotein but did not significantly alter Runx2 levels. Matrix mineralization also occurred earlier in HDAC3-suppressed cells, but alkaline phosphatase expression was not affected. Thus, HDAC3 regulates osteoblast differentiation and bone formation. Although HDAC3 is likely to affect the activity of multiple proteins in osteoblasts, our data show that it actively regulates the transcriptional activity of the osteoblast master protein, Runx2.

Runx2 induces osteoblast and chondrocyte differentiation and enhances their migration by coupling with PI3K-Akt signaling

Runx2 and phosphatidylinositol 3-kinase (PI3K)–Akt signaling play important roles in osteoblast and chondrocyte differentiation. We investigated the relationship between Runx2 and PI3K-Akt signaling. Forced expression of Runx2 enhanced osteoblastic differentiation of C3H10T1/2 and MC3T3-E1 cells and enhanced chondrogenic differentiation of ATDC5 cells, whereas these effects were blocked by treatment with IGF-I antibody or LY294002 or adenoviral introduction of dominant-negative (dn)–Akt. Forced expression of Runx2 or dn-Runx2 enhanced or inhibited cell migration, respectively, whereas the enhancement by Runx2 was abolished by treatment with LY294002 or adenoviral introduction of dn-Akt. Runx2 up-regulated PI3K subunits (p85 and p110β) and Akt, and their expression patterns were similar to that of Runx2 in growth plates. Treatment with LY294002 or introduction of dn-Akt severely diminished DNA binding of Runx2 and Runx2-dependent transcription, whereas forced expression of myrAkt enhanced them. These findings demonstrate that Runx2 and PI3K-Akt signaling are mutually dependent on each other in the regulation of osteoblast and chondrocyte differentiation and their migration.

Mechanical signals as regulators of stem cell fate

In recent years, stem cells have shown significant promise for their potential to provide a source of undifferentiated progenitor cells for therapeutic applications in tissue or organ repair. Significant questions still remain, however, as to the genetic and epigenetic signals that regulate the fate of stem cells. It is now well accepted that the micro-environment of the stem cell can have a significant influence on its differentiation and phenotypic expression. Although emphasis has been placed in previous work on the role of soluble mediators such as growth factors and cytokines on stem cell differentiation, there is now significant evidence, both direct and indirect, that mechanical signals may also regulate stem cell fate. We review a number of in vivo and in vitro studies that have provided evidence that mechanical factors have the ability to influence the differentiation of a number of cells that have been classified as either precursor, progenitor, or stem cells. Taken together, these studies show that specific mechanical signals may promote cell differentiation into a particular phenotype, potentially having an effect on embryonic development. The use of such mechanical signals in vitro in specially designed "bioreactors" may provide important adjuncts to standard biochemical signaling pathways for promoting engineered tissue growth. A further understanding of the biomechanical and biochemical pathways involved in mechanical signal transduction by stem cells will hopefully provide new insight for the improvement of stem-cell based therapies.

Identification of novel regulators associated with early-phase osteoblast differentiation

Key regulatory components of the BMP-induced osteoblast differentiation cascade remain to be established. Microarray and subsequent expression analyses in mice identified two transcription factors, Hey1 and Tcf7, with in vitro and in vivo expression characteristics very similar to Cbfa1. Transfection studies suggest that Tcf7 modulates BMP2-induced osteoblast differentiation. This study contributes to a better definition of the onset of BMP-induced osteoblast differentiation.

INTRODUCTION

Elucidation of the genetic cascade guiding mesenchymal stem cells to become osteoblasts is of extreme importance for improving the treatment of bone-related diseases such as osteoporosis. The aim of this study was to identify regulators of the early phases of bone morphogenetic protein (BMP)2-induced osteoblast differentiation.

MATERIALS AND METHODS

Osteoblast differentiation of mouse C2C12 cells was induced by treatment with BMP2, and regulation of gene expression was studied during the subsequent 24 h using high-density microarrays. The regulated genes were grouped by means of model-based clustering, and protein functions were assigned. Real-time quantitative RT-PCR analysis was used to validate BMP2-induced gene expression patterns in C2C12 cells. Osteoblast specificity was studied by comparing these expression patterns with those in C3H10T1/2 and NIH3T3 cells under similar conditions. In situ hybridization of mRNA in embryos at embryonic day (E)14.5 and E16.5 of gestation and on newborn mouse tails were used to study in vivo expression patterns. Cells constitutively expressing the regulated gene Tcf7 were used to investigate its influence on BMP-induced osteoblast differentiation.

RESULTS AND CONCLUSIONS

A total of 184 genes and expressed sequence tags (ESTs) were differentially expressed in the first 24 h after BMP2 treatment and grouped in subsets of immediate early, intermediate early, and late early response genes. Signal transduction regulatory factors mainly represented the subset of immediate early genes. Regulation of expression of these genes was direct, independent of de novo protein synthesis and independent of the cell type studied. The intermediate early and late early genes consisted primarily of genes related to processes that modulate morphology, basement membrane formation, and synthesis of extracellular calcified matrix. The late early genes require de novo protein synthesis and show osteoblast specificity. In vivo and in vitro experiments showed that the transcription factors Hey1 and Tcf7 exhibited expression characteristics and cell type specificity very similar to those of the osteoblast specific transcription factor Cbfa1, and constitutive expression of Tcf7 in C2C12 cells differentially regulated osteoblast differentiation marker genes.

Biomimetic peptides that engage specific integrin-dependent signaling pathways and bind to calcium phosphate surfaces

Many important matrix proteins involved in bone remodeling contain separate domains that orient the protein on hydroxyapatite and interact with target cell receptors, respectively. We have designed two synthetic peptides that mimic the dual activities of these large, complex proteins by binding to calcium phosphate minerals and by engaging integrin-dependent signaling pathways in osteoblasts. The addition of either PGRGDS from osteopontin or PDGEA from collagen type I to the HAP-binding domain of statherin (N15 domain) did not alter its alpha-helical structure or diminish its affinity for hydroxyapatite. Immobilized N15-PGRGDS bound MC3T3-E1 osteoblasts predominantly via the alpha v beta 3 integrin and induced focal adhesion kinase (FAK) phosphorylation at comparable levels to immobilized osteopontin. Immobilized N15-PDGEA bound MC3T3-E1 osteoblasts predominantly through the alpha 2 beta 1 integrin and induced similar levels of FAK phosphorylation. Although both peptides induced FAK phosphorylation with similar time courses, only the N15-PDGEA peptide induced ERK1/2 phosphorylation, showing that these peptides are also capable of engaging integrin-specific signaling pathways. This peptide system can be used to study adhesion-dependent control of signaling in the context of the relevant biomineral surface and may also be useful in biomaterial and tissue engineering applications.

MINT, the Msx2 interacting nuclear matrix target, enhances Runx2-dependent activation of the osteocalcin fibroblast growth factor response element

Msx2 promotes osteogenic lineage allocation from mesenchymal progenitors but inhibits terminal differentiation demarcated by osteocalcin (OC) gene expression. Msx2 inhibits OC expression by targeting the fibroblast growth factor responsive element (OCFRE), a 42-bp DNA domain in the OC gene bound by the Msx2 interacting nuclear target protein (MINT) and Runx2/Cbfa1. To better understand Msx2 regulation of the OCFRE, we have studied functional interactions between MINT and Runx2, a master regulator of osteoblast differentiation. In MC3T3E1 osteoblasts (with endogenous Runx2 and FGFR2), MINT augments transcription driven by the OCFRE that is further enhanced by FGF2 treatment. OCFRE regulation can be reconstituted in the naïve CV1 fibroblast cell background. In CV1 cells, MINT synergizes with Runx2 to enhance OCFRE activity in the presence of activated FGFR2. The RNA recognition motif domain of MINT (which binds the OCFRE) is required. Runx2 structural studies reveal that synergy with MINT uniquely requires Runx2 activation domain 3. In confocal immunofluorescence microscopy, MINT adopts a reticular nuclear matrix distribution that overlaps transcriptionally active osteoblast chromatin, extensively co-localizing with the phosphorylated RNA polymerase II meshwork. MINT only partially co-localizes with Runx2; however, co-localization is enhanced 2.5-fold by FGF2 stimulation. Msx2 abrogates Runx2-MINT OCFRE activation, and MINT-directed RNA interference reduces endogenous OC expression. In chromatin immunoprecipitation assays, Msx2 selectively inhibits Runx2 binding to OC chromatin. Thus, MINT enhances Runx2 activation of multiprotein complexes assembled by the OCFRE. Msx2 targets this complex as a mechanism of transcriptional inhibition. In osteoblasts, MINT may serve as a nuclear matrix platform that organizes and integrates osteogenic transcriptional responses.

Selective Deficiency of the “Bone-related” Runx2-II Unexpectedly Preserves Osteoblast-mediated Skeletogenesis

Runx2 (runt-related transcription factor 2) is a master regulator of skeletogenesis. Distinct promoters in the Runx2 gene transcribe the "bone-related" Runx2-II and non-osseous Runx2-I isoforms that differ only in their respective N termini. Existing mutant mouse models with both isoforms deleted exhibit an arrest of osteoblast and chondrocyte maturation and the complete absence of mineralized bone, but they do not distinguish the separate functions of the two N-terminal isoforms. To elucidate the function of the bone-related isoform, we generated selective Runx2-II-deficient mice by the targeted deletion of the distal promoter and exon 1. Homozygous Runx2-II-deficient (Runx2-II(-/-)) mice unexpectedly formed axial, appendicular, and craniofacial bones derived from either intramembranous ossification or mesenchymal cells of the bone collar, but they failed to form the posterior cranium and other bones derived from endochondral ossification. Heterozygous Runx2-II-deficient mice had grossly normal skeletons, but were osteopenic. The commitment of mesenchymal cells ex vivo to the osteoblast lineage occurred in Runx2-II(-/-) mice, but osteoblastic gene expression was impaired. Chondrocyte maturation appeared normal, but the zone of hypertrophic chondrocytes was not transformed into metaphyseal bone, leading to widened growth plates in Runx2-II(-/-) mice. Compensatory increments in Runx2-I expression occurred in Runx2-II(-/-) mice but were not sufficient to normalize osteoblastic maturation or transcriptional activity. Our findings support distinct functions of Runx2-II and -I in the control of skeletogenesis. Runx2-I is sufficient for early osteoblastogenesis and intramembranous bone formation, whereas Runx2-II is necessary for complete osteoblastic maturation and endochondral bone formation.

Modeled microgravity inhibits osteogenic differentiation of human mesenchymal stem cells and increases adipogenesis

Space flight-induced bone loss has been attributed to a decrease in osteoblast function, without a significant change in bone resorption. To determine the effect of microgravity (MG) on bone, we used the Rotary Cell Culture System [developed by the National Aeronautics and Space Administration (NASA)] to model MG. Cultured mouse calvariae demonstrated a 3-fold decrease in alkaline phosphatase (ALP) activity and failed to mineralize after 7 d of MG. ALP and osteocalcin gene expression were also decreased. To determine the effects of MG on osteoblastogenesis, we cultured human mesenchymal stem cells (hMSC) on plastic microcarriers, and osteogenic differentiation was induced immediately before the initiation of modeled MG. A marked suppression of hMSC differentiation into osteoblasts was observed because the cells failed to express ALP, collagen 1, and osteonectin. The expression of runt-related transcription factor 2 was also inhibited. Interestingly, we found that peroxisome proliferator-activated receptor gamma (PPARgamma2), which is known to be important for adipocyte differentiation, adipsin, leptin, and glucose transporter-4 are highly expressed in response to MG. These changes were not corrected after 35 d of readaptation to normal gravity. In addition, MG decreased ERK- and increased p38-phosphorylation. These pathways are known to regulate the activity of runt-related transcription factor 2 and PPARgamma2, respectively. Taken together, our findings indicate that modeled MG inhibits the osteoblastic differentiation of hMSC and induces the development of an adipocytic lineage phenotype. This work will increase understanding and aid in the prevention of bone loss, not only in MG but also potentially in age-and disuse-related osteoporosis.

Regulation of TGFβ1-mediated growth inhibition and apoptosis by RUNX2 isoforms in endothelial cells

Runx transcription factors regulate viral growth, hematopoiesis, bone formation, angiogenesis, and gastric epithelial development through specific DNA-binding motifs on target gene promoters. Vascular endothelial cells (ECs) express RUNX genes that are activated by angiogenic factors. The RUNX2 gene also activates the vascular endothelial growth factor promoter. Alternatively spliced forms of RUNX genes have been described, but their functions in angiogenesis have not been elucidated. In this study, expression of a novel alternatively spliced variant of RUNX2 (RUNX2Delta8), lacking the region encoded by exon 8, was detected in aortic tissue undergoing angiogenesis in vitro and in ECs. Expression of RUNX2 and RUNX2Delta8 increased in vascular sprouts concomitant with expression of cellular proteases and cytokines known to mediate angiogenesis. RUNX2 DNA-binding activity was expressed in proliferating but not quiescent ECs. Ectopic expression of RUNX2 in ECs increased cell sprouting, cell proliferation, DNA synthesis, and phosphorylation of phosphorylated retinoblastoma relative to control transfectants while RUNX2, but not RUNX2Delta8 transfectants, acquired resistance to growth inhibition by transforming growth factor (TGFbeta1). Furthermore, RUNX2Delta8-transfected cells were more sensitive to TGFbeta1-induced apoptosis than RUNX2 transfectants. Consistent with these data, the RUNX2 gene was a strong repressor of the promoter of the cyclin-dependent kinase inhibitor, p21(CIP1), while RUNX2Delta8 was a competitive inhibitor of RUNX2 and exhibited weak repression activity. These results support the hypothesis that ECs regulate growth and apoptosis, in part, by alternative splicing events in the RUNX2 transcription factor to affect the TGFbeta1 signaling pathway. The exon 8 domain of RUNX2 may contribute to the strong repression activity of RUNX2 for some target gene promoters.

α2β1integrin-specific collagen-mimetic surfaces supporting osteoblastic differentiation

The interactions of osteoblasts with their surrounding extracellular matrix (ECM) are essential for skeletal development, homeostasis, and maintenance of the mature osteoblastic phenotype. Integrins are the principal transducers of ECM signals that regulate this process of osteoblast commitment and differentiation. Several studies indicate that the alpha(2)beta(1) integrin interaction with type I collagen is a crucial signal for the induction of osteoblastic differentiation and matrix mineralization. Integrin alpha(2)beta(1) recognizes the Gly-Phe-Hyp-Gly-Glu-Arg (GFOGER) motif in residues 502-507 of the alpha(1)[I] chain of type I collagen. This study demonstrates that an alpha(2)beta(1) integrin-specific GFOGER peptide triggers the activation of focal adhesion kinase and alkaline phosphatase in MC3T3-E1 murine immature osteoblast-like cells, two events that have been implicated in the osteoblastic differentiation pathway. These GFOGER-peptide surfaces also support the expression of multiple osteoblast-specific genes, including osteocalcin and bone sialoprotein, and induce matrix mineralization in a manner similar to type I collagen. This triple-helical peptide represents a promising surface modification strategy for the design of collagen-mimetic bioadhesive surfaces that support osteoblastic differentiation.

Tumor promoter-induced MMP-13 gene expression in a model of initiated epidermis

In mouse epidermis in vivo, the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) increases gene expression of matrix metalloproteinase-13 (MMP-13), an enzyme implicated in carcinogenesis. Here we used a keratinocyte cell line (308) derived from initiated mouse skin to investigate TPA-induced MMP-13 gene expression. Use of a pharmacological inhibitor (U0126) demonstrated that extracellular signal regulated kinase (ERK) plays a major role in TPA-induced MMP-13 gene expression. The 5'-flanking sequences of the MMP-13 gene contain binding sites for activator protein-1 (AP-1) and Runx. Both transcription factor families can be modulated by ERK and have been implicated in MMP-13 gene expression. TPA stimulated ERK-dependent increases in c-Fos protein and the c-Fos content of AP-1 complexes. MMP-13 promoter studies indicated that TPA requires AP-1, but not Runx, to induce MMP-13 gene expression. These studies show that in mouse keratinocytes MMP-13 gene expression can be induced through a Runx-independent pathway that involves the ERK-dependent modulation of AP-1.

Modified U1 snRNA suppresses expression of a targeted endogenous RNA by inhibiting polyadenylation of the transcript

We have previously demonstrated that a modified U1 snRNA inhibits expression of a number of targeted transgenes. Here we exploit the ability of the modified U1 snRNA to inhibit endogenous gene expression and define the mechanism responsible for this inhibitory action. MC3T3-E1 cells stably transfected with U1 anti-Cbfa1 show a change of morphology from polygonal to fibroblast-like cells. This visual observation was supported by an 80% reduction of Cbfa1 expression and suppression of downstream genes associated with osteoblast differentiation. In rat ROS 17/2.8 cells, osteocalcin and Col1a1 gene expression was reduced up to 90% by the U1 anti-osteocalcin or U1 anti-Col1a1 construct, respectively. The length of mature osteocalcin mRNA poly(A) tail was significantly shortened in the targeted mRNA by transcript-specific poly(A) test (PAT)-PCR. These data demonstrate that the modified U1 snRNA is able to reduce endogenous gene expression by limiting the polyadenylation of the targeted pre-mRNA transcript.

Physical stress by magnetic force accelerates differentiation of human osteoblasts

We examined the effect of magnetic force on differentiation of cultured human osteoblasts. Magnetic microparticles (MPs) were introduced into the cytoplasm of a human osteoblast cell line and the cells were cultured in a magnetic field (MF) in group MP-MF. Three groups of controls were used: cells without MPs were cultured out of MF (group C), cells without MPs were cultured in MF (group MF), and cells with MPs were cultured out of MF (group MP). The cells in group MP-MF became larger and were elongated along the axis of the magnetic poles. Appearance of alkaline phosphatase (AlPase) activity, formation of bone nodules, and calcium deposition were accelerated depending on the intensity of the magnetic field. It takes longer culture in the other three groups to exhibit these changes. Core-binding factor A1 (Cbfa1: transcription factor for osteoblast differentiation) and osteocalcin (a bone-matrix protein involved in controlling osteogenesis) were expressed earlier or stronger in group MP-MF than the other groups. Then we compared phosphorylation of mitogen-activated protein kinase (MAPK) between group MP-MF and group C. Phosphorylation of p38(MAPK) (p38) was increased in group MP-MF, while total p38 as well as total and phosphorylated forms of MAPK/ERK 1/2 and SAPK/JNK were not changed between the two groups. When a p38 inhibitor, SB 203580, was added to the culture medium in group C, AlPase activity, formation of bone nodules, and calcium deposits were completely inhibited. On the other hand, they were inhibited only partially by a MAPK/ERK 1/2 inhibitor, U-0126. Based on these results, it is concluded that (1) osteoblast differentiation is accelerated by a magnetic force, (2) this acceleration is mainly attributed to the activation of p38 phosphorylation, and (3) the stimulus induced by a magnetic field offers a new approach to osteoblast differentiation.

Differential effects of PD98059 and U0126 on osteogenesis and adipogenesis

PD98059 and U0126 are considered as specific inhibitors of the p42/44 mitogen-activated protein kinases (MAPK) pathway, which affects osteogenesis and adipogenesis. Here, we show unexpected differential effects of PD98059 and U0126 on osteogenesis and adipogenesis as well as on estrogen (E2)-induced actions in osteoprogenitor KS483 cells. PD98059 dose-dependently inhibited osteogenesis indicated by cellular alkaline phosphatase (ALP) activity and nodule formation, but stimulated adipogenesis shown by the number of adipocytes. In contrast, U0126 slightly decreased osteogenesis but had no effects on adipogenesis, although it inhibited p42/44 MAPK more potently than PD98059. Furthermore, PD98059, but not U0126, counteracted E2-induced osteogenesis and adipogenesis. Transfection experiments showed that PD98059, but not U0126, had estrogenic transcriptional activity. Interestingly, both PD98059 and U0126 potentiated E2-induced estrogenic transcriptional activity in KS483 cells, which is opposite to the response in MCF7 breast cancer cells. Our data indicate that the cross-talk between growth factors and estrogen receptor (ER)-mediated pathways in KS483 cells is different from that in MCF7 cells. In summary, the differential effects of PD98059 and U0126 indicate their actions are not exclusively due to an inhibition of MAPK pathway. Caution should be taken in the interpretation of the results obtained using these inhibitors.

Type I collagen-mediated changes in gene expression and function of prostate cancer cells

In this study, cDNA microarrays were used to characterize gene expression changes elicited in prostate cancer cells by plating them on type I collagen. The results clearly reveal changes in the expression of genes associated with cellular signaling, cellular metabolism, gene transcription and gene translation which are indicative of cells that are actively proliferating. Together these results suggest that these changes in the gene expression profiles mediated by type I collagen may influence the proliferative capacity of prostate cancer cells in the bone microenvironment and facilitate development of prostate cancer bone metastases. Additionally, the microarray approach provides an invaluable tool to determine and track changes in gene expression in numerous disease states including prostate cancer. This technology is certain to facilitate discovery of new therapeutic gene targets.

Modulation of prostate cancer growth in bone microenvironments

Bone remains one of the major sites, and most lethal host organs, for prostate cancer metastasis. Prostate cell spread and establishment in bone depends on multiple reciprocal modifications of bone stromal and epithelial cancer cell behaviors. This review focuses on recent advances in the characterization of cell-cell and cell-matrix interplay, effects on cell growth, adhesion and invasion, and several therapeutic possibilities for co-targeting prostate cancer cells and bone stroma. We address the topic from three main perspectives: (1) the normal and aging bone stromal environment, (2) the "reactive" bone stromal environment, and (3) the cancerous prostate epithelial cells themselves. First, normal, and especially aging, bones provide uniquely rich and "fertile soil" for roaming cancer cells. The interactions between prostate cancer cells and insoluble extracellular matrices, soluble growth factors, and/or sex steroid hormones trigger bone remodeling, through increased osteoclastogenesis and furthur matrix metalloproteinase activity. Second, after cancer cell arrival and establishment in the bone, host stromal cells respond, becoming "reactive" in a process again involving extracellular matrix remodeling, together with growth factor and steroid receptor signaling this process ultimately enhances cancer cell migration, stromal transdifferentiation, and invasion of the cancer tissues by stromal, inflammatory, and immune-responsive cells. Third, prostate cancer cells also respond to supportive bone microenvironments, where soluble and matrix-associated molecules affect cancer cell growth and gene expression, especially altering cancer cell surface receptor and integrin-mediated cell signaling. We discuss both integrin cell-matrix and gap junctional cell-cell communication between cancer cells and their microenvironments during prostate cancer progression.

Dexamethasone inhibits insulin-induced chondrogenesis of ATDC5 cells by preventing PI3K-Akt signaling and DNA binding of Runx2

Glucocorticoids play important roles in cell growth and differentiation. In this study, we investigated the effect of application of dexamethasone (DEX) at the early stage of chondrogenesis using the prechondrogenic cell line, ATDC5, which differentiates into chondrocytes in the presence of insulin. When ATDC5 cells were cultured in the presence of DEX and insulin, DEX inhibited insulin-induced cellular condensation and subsequent cartilaginous nodule formation, and reduced proteoglycan synthesis and type II collagen expression dose-dependently. Pretreatment with 10(-8) M DEX for 1 day inhibited insulin-induced Akt phosphorylation, but not ERK1/2 phosphorylation, in ATDC5 cells. Treatment of ATDC5 cells with insulin for more than 2 days upregulated the levels of phosphatidylinositol 3-kinase (PI3K) subunit proteins, p85 and p110, and Akt, whereas the upregulation was inhibited in the presence of 10(-8) M DEX. In electrophoresis mobility shift assays (EMSAs), treatment with 10(-8) M DEX inhibited DNA binding of Runx2 during culture of ATDC5 cells with insulin. Reporter assays using osteocalcin promoter showed that DEX inhibited Runx2-dependent transcription dose-dependently. Adenoviral introduction of dominant-negative (dn)-Akt or dn-Runx2 into ATDC5 cells inhibited cellular condensation and reduced proteoglycan synthesis upon incubation with insulin, whereas adenoviral introduction of Akt or Runx2 prevented the inhibition of chondrogenesis by DEX. These findings indicate that DEX inhibits chondrogenesis of ATDC5 cells at the early stage by downregulating Akt phosphorylation as well as the protein levels of PI3K subunits and Akt, thereby suppressing PI3K-Akt signaling, and by inhibiting DNA binding of Runx2 and Runx2-dependent transcription.

Notch signaling and ERK activation are important for the osteomimetic properties of prostate cancer bone metastatic cell lines

Prostate cancer bone metastases are characterized by their ability to induce osteoblastic lesions and local bone formation. It has been suggested that bone metastatic prostate cancer cells are osteomimetic and capable of expressing genes and proteins typically expressed by osteoblasts. The ability of preosteoblasts to differentiate and express osteoblastic genes depends on several pathways, including Notch and MAPK. Here we show that notch1 expression is increased 4-5 times in C4-2B and MDA PCa 2b cells (osteoblastic skeletal prostate metastatic cancer cell lines) when compared with nonskeletal metastatic cell lines (LNCaP and DU145). Notch1 ligand, dll1, is expressed only in C4-2B cells. Immunohistochemical studies demonstrate that Notch1 is present in both human clinical samples from prostate cancer bone metastases and the C4-2B cell line. To determine whether prostate cancer bone metastases respond to osteogenic induction similar to osteoblasts, C4-2B cells were cultured in osteogenic medium that promotes mineralization. C4-2B cells mineralize and express HES-1 (a downstream target of Notch), an effect that is completely inhibited by L-685,458, a Notch activity inhibitor. Furthermore, osteogenic induction increases ERK activation, runx2 expression, and nuclear localization, independent of Notch signaling. Finally, we show that Notch and ERK activation are essential for Runx2 DNA binding activity and osteocalcin gene expression in C4-2B cells in response to osteogenic induction. These studies demonstrate that prostate cancer bone metastatic cell lines acquire osteoblastic properties through independent activation of ERK and Notch signaling; presumably, both pathways are activated in the bone microenvironment.

Transcriptional regulation by the MAP kinase signaling cascades

A major function of the mitogen-activated protein kinase (MAPK) pathways is to control eukaryotic gene expression programmes in response to extracellular signals. MAPKs directly control gene expression by phosphorylating transcription factors. However, it is becoming clear that transcriptional regulation in response to MAPK signaling is more complex. MAPKs can also target coactivators and corepressors and affect nucleosomal structure by inducing histone modifications. Furthermore, multiple inputs into individual promoters can be elicited by MAPKs by targeting different components of the same coregulatory complex or by triggering different events on the same transcription factor. "Postgenomic approaches" are beginning to impact on our understanding of these gene regulatory networks. In this review, we summarise the current knowledge of MAPK-mediated gene regulation, and focus on how complexities in signaling outcomes are achieved and how this relates to physiological processes.

Fibroblast growth factor signaling controlling osteoblast differentiation

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

Cyclic strain enhances matrix mineralization by adult human mesenchymal stem cells via the extracellular signal-regulated kinase (ERK1/2) signaling pathway

Physical stimuli play critical roles in the development, regeneration, and pathology of many mesenchymal tissues, most notably bone. While mature bone cells, such as osteoblasts and osteocytes, are clearly involved in these processes, the role of their progenitors in mechanically mediated tissue responses is unknown. In this study, we investigated the effect of cyclic substrate deformation on the proliferation and osteogenic differentiation of human mesenchymal stem cells (hMSCs). Application of equibiaxial cyclic strain (3%, 0.25Hz) to hMSCs cultured in osteogenic media inhibited proliferation and stimulated a 2.3-fold increase in matrix mineralization over unstrained cells. The strain stimulus activated the extracellular signal-regulated kinase (ERK1/2) and p38 mitogen-activated protein kinase pathways, but had no effect on c-Jun N-terminal kinase phosphorylation or activity. Strain-induced mineralization was largely mediated by ERK1/2 signaling, as inhibition of ERK1/2 attenuated calcium deposition by 55%. Inhibition of the p38 pathway resulted in a more mature osteogenic phenotype, suggesting an inhibitory role for p38 signaling in the modulation of strain-induced osteogenic differentiation. These results demonstrate that mechanical signals regulate hMSC function, suggesting a critical role for physical stimulation of this specific cell population in mesenchymal tissue formation.

Transcriptional regulation of the human Runx2/Cbfa1 gene promoter by bone morphogenetic protein-7

It is well established that core binding factor Runx2/Cbfa1 is required for osteoblast recruitment and differentiation from mesenchymal stem cells. Transcriptional regulation of the Runx2/Cbfa1 gene by osteogenic factors such as bone morphogenetic proteins (BMPs) plays an important role in the stimulation of bone formation by these cytokines. BMP7 (also termed OP-1) is a member of the transforming growth factor beta (TGF-beta) superfamily and induces osteoblast differentiation from mesenchymal precursor stem cells in vitro as well as bone formation in vivo. This study examines the effects of BMP7 on markers of osteoblast differentiation and specifically on human Runx2/Cbfa1 gene transcription in a mouse C2C12 myoblast cell line where it induces expression of both alkaline phosphatase (ALP) and endogenous Runx2/Cbfa1. To further understand the mechanisms of human Runx2/Cbfa1 transcriptional regulation by BMP7, we cloned 3.0 kb of the human Runx2/Cbfa1 gene 5'-upstream flanking region and created a series of promoter deletions cloned into luciferase-based reporter vectors (Runx2/Cbfa1/Luc). Sequence data revealed six copies of the osteoblastic cis-acting element (OSE2) in the proximal promoter region. In C2C12 cells transiently transfected with Runx2/Cbfa1/Luc deletion constructs, transcriptional activity of Runx2/Cbfa1 was upregulated up to 2-fold after 24 h of BMP7 treatment. Mutational analysis demonstrated that the minimal responsive promoter region for BMP7-regulated transcription maps to a proximal -74 OSE2 site. Electromobility shift assays with C2C12 cellular extracts indicate that BMP7 increases binding of OSE2 promoter sequences, and supershift assays with anti-Runx2/Cbfa1 antibodies demonstrate that Runx2/Cbfa1 is part of the nucleoprotein complex binding OSE2. Together, these data indicate BMP7 can upregulate Runx2/Cbfa1 gene expression in C2C12 myoblast cells, and suggest that Runx2/Cbfa1 may bind to OSE2 elements within its own promoter to autoregulate gene transcription in differentiating osteoblasts.

Cot/Tpl2 is essential for RANKL induction by lipid A in osteoblasts

Lipopolysaccharide (LPS) is a pathogenic factor that increases bone resorption in periodontal diseases. LPS treatment of osteoblasts was shown to induce the receptor activator of NF-kappa B ligand (RANKL), an essential secretory or membrane-bound factor for osteoclast function, in a manner dependent on extracellular signal-regulated kinase (ERK) activation. However, the mechanisms regulating this process remained unknown. Here, we show that RANKL mRNA induction and ERK activation, when treated with synthetic lipid A (an active center of LPS), were markedly reduced in mouse osteoblasts lacking Cot/Tpl2, which was recently recognized as an essential kinase for the induction of TNF-alpha by LPS in macrophages. In contrast, c-Jun N-terminal kinase (JNK), p38 kinase, Raf-1, and NF-kappa B were normally activated in cot/tpl2-/- osteoblasts. These findings indicate that Cot/Tpl2 is essential for LPS-induced ERK activation and RANKL induction in osteoblasts.

Prostate cancer expression of runt-domain transcription factor Runx2, a key regulator of osteoblast differentiation and function

BACKGROUND

Prostate cancer (CaP) bone metastases express numerous proteins associated with bone cells. Specific transcription factors, including Runx2, regulate the expression of many bone-related factors in osteoblasts. Expression of these transcription factors in CaP may be linked to the ability of CaP bone metastases to influence bone remodeling.

METHODS

CaP tissues and cell lines were analyzed for expression of Runx2 mRNA by RT-PCR and in situ hybridization, and protein by immunohistochemistry, Western blotting, and electrophoretic mobility shift assays (EMSA).

RESULTS

Runx2 mRNA and protein were detected in CaP tissues and cell lines. A specific Runx2: OSE2 complex could be formed with PC-3 nuclear extracts.

CONCLUSIONS

Expression of Runx2 in CaP may be the molecular switch that is associated with expression of various bone-specific factors in CaP. In turn, expression of these factors can influence bone remodeling and possibly play a role in the growth and survival of CaP in bone.

Bone tissue engineering: recent advances and promising therapeutic agents

Bone regeneration can be accomplished with growth factors, cells and delivery systems. This review is a summary of these components that may be used for tissue regeneration. Support for the potential therapeutic applications of transcription factors in bone tissue engineering will also be discussed.

Expression of galectin-3 in skeletal tissues is controlled by Runx2

The beta-galatoside-specific lectin galectin-3 is expressed in vivo in osteoblasts as well as in epiphyseal cartilage. Here we show that in vitro, galectin-3 expression is up-regulated in the preosteoblastic cell line MC3T3-E1 during the matrix maturation stage of the osteoblast developmental sequence. Expression persists into late differentiation stages when the mature osteoblastic phenotype is established. The skeletal expression pattern of galectin-3 overlaps at many sites with that of the transcription factor Runx2. Runx2 is a key regulator of osteoblast development and necessary for chondrocyte differentiation in the growth plate. Both human and mouse galectin-3 promoters contain putative Runx-binding sites. The constitutive or inducible forced expression of Runx2 is sufficient for the onset of galectin-3 transcription in the mesenchymal precursor cell line C3H10T1/2. Moreover, Runx2 is able to bind to at least two sites in the galectin-3 promoter region. The crucial role of Runx2 was confirmed in Runx2-deficient mice, which are devoid of galectin-3 expression in skeletal cells. The overlapping expression pattern of galectin-3 with the other two members of the Runt family of transcription factors (Runx1 and Runx3) points to a potential regulation of the galectin-3 gene (LGALS3) by these factors in hematopoietic, skin, and dorsal root ganglial cells.

Receptor tyrosine kinases inhibit bone morphogenetic protein-Smad responsive promoter activity and differentiation of murine MC3T3-E1 osteoblast-like cells

Growth factors such as fibroblast growth factor-2 (FGF-2) and epidermal growth factor (EGF) that activate extracellular signal-regulated kinases (ERKs) through receptor tyrosine kinases (RTKs) stimulate proliferation but suppress differentiation of osteoblasts. To study the mechanism of this inhibitory action of these growth factors on osteoblastic differentiation, we evaluated Smad1 transactivity in MC3T3-E1 osteoblast-like cells by reporters of promoter activity of mouse Smad6, an early response gene to bone morphogenetic proteins (BMPs). FGF-2 and EGF inhibited alkaline phosphatase activity and Smad6 promoter activity stimulated by BMP-2. Overexpression of constitutively active MEK by adenovirus mimicked, but that of dominant negative Ras or treatment with a MEK1 inhibitor, PD098059, reversed, the inhibitory effects of these growth factors on both activities. These effects are mediated by BMP-responsive elements (BMPREs) on Smad6 promoter, because an artificial reporter driven by three tandem BMPREs gave similar results, and these effects were all abolished when the BMPREs were mutated. RTK-ERK activation inhibited the promoter activity even when BMP signal was mediated by a mutant Smad1, which lacks phosphorylation sites by ERKs, or by a Smad1 fused to Gal4 DNA binding domain, which constitutively localizes in the nucleus. These results show that the RTK-Ras-ERK pathway suppresses BMP signal by interfering with Smad1 transactivity. Because direct phosphorylation of Smad1 by ERKs is not required for the inhibition, other transcriptional factors that are phosphorylated by ERKs might be involved in the regulation of osteoblastic differentiation by ERKs.

Okadaic acid stimulates osteopontin expression through de novo induction of AP-1

Osteopontin, a major non-collagenous bone matrix protein, is strikingly upregulated in various tissues under certain pathologic conditions, including cancer. However, the mechanism of upregulation of the osteopontin gene in tumor cells remains unclear. Okadaic acid, a strong non-phorbol ester tumor promoter, is known to stimulate the expression of osteopontin. The aim of the present study was to understand the mechanism by which okadaic acid regulates osteopontin gene expression. Okadaic acid stimulated osteopontin mRNA expression in several cell lines within 3 h, and the increase in osteopontin mRNA was sustained for 24 h. New protein synthesis was required for the okadaic acid-elicited increase in osteopontin mRNA expression. A serial promoter deletion study showed that the okadaic acid-response element is located between positions -265 and -73, a sequence that includes the Runx2, Ets-1, and AP-1 binding sequences. Okadaic acid increased the mRNA expression of AP-1 components but not of Runx2 or Ets-1. Site-directed mutagenesis and electrophoretic mobility shift assays confirmed that protein binding of the AP-1 consensus sequence is necessary for the okadaic acid-mediated osteopontin gene upregulation. These results indicate that de novo induction of the oncoprotein AP-1 is required for okadaic acid-stimulated osteopontin gene upregulation.

Runx2, a multifunctional transcription factor in skeletal development

The identification of Runx2 (runt-related protein 2) function has greatly advanced the understanding of skeletal development over the last 5 years. Runx2 is regulated transcriptionally and post-translationally through the activity of many identified factors, although, the physiological significance of each remains to be demonstrated. The interaction of Runx2 with other transcription factors and cofactors has been shown to be important in Runx2-dependent gene regulation. Runx2 plays important roles in multiple steps of skeletal development. Runx2 determines the lineage of osteoblasts from multipotent mesenchymal cells, enhances osteoblast differentiation at an early stage, and inhibits osteoblast differentiation at a late stage. Runx2 plays crucial roles in chondrocyte maturation and in the specification of cartilage phenotypes. Furthermore, Runx2 is involved in vascular invasion into cartilage and osteoclastogenesis. Therefore, the determination of Runx2 function and the investigation of the cascades of Runx2-dependent gene regulation are important in the elucidation of skeletal biology.

Type I collagen regulated dentin matrix protein-1 (Dmp-1) and osteocalcin (OCN) gene expression of rat dental pulp cells

In this study, we investigated the effect of type I collagen on dentin matrix protein-1 (Dmp-1) and osteocalcin (OCN) gene expression of dental pulp cells. The mRNA level of Dmp-1 gene was down-regulated; however, OCN gene expression was up-regulated by the culture of dental pulp cells with type I collagen. These findings imply that type I collagen regulates mRNA level of Dmp-1 and OCN gene that are predominantly expressed in active odontoblasts. The change of gene expression by type I collagen was suppressed by the blocking of collagen-integrin interaction. We could conclude that the effect of type I collagen was mediated via binding of collagen to integrin receptors.

Bone development in neurofibromatosis 1

Bony abnormalities are common findings in cases of neurofibromatosis 1. We might hypothesize that neurofibromin, the protein encoded by the neurofibromatosis 1 gene, plays important roles in bone development. Loss of function of oligodendrocyte-myelin glycoprotein gene and increased activity of ras p21 might increase the level of c-fos proto-oncogene in bones with formation of fibrous dysplasia-like tissue. Also, increased ras p21 might disturb collagen I synthesis by osteoblasts. Moreover, increased ras activity might increase the mitogenic signals to the nucleus through mitogen-activated protein kinase (MAPK) and disturb the level of the transcription factor core-binding factor alpha(1) (Cbfa1). Abnormal fibrous tissue and neurofibromas formed at the site of pseudarthrosis might represent abnormal response of periosteal fibroblasts for injury, an effect simulating the response of skin fibroblasts in neurofibromatosis 1 to injury.

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

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

Simulated microgravity suppresses osteoblast phenotype, Runx2 levels and AP-1 transactivation

Conditions of disuse such as bed rest, space flight, and immobilization result in decreased mechanical loading of bone, which is associated with reduced bone mineral density and increased fracture risk. Mechanisms involved in this process are not well understood but involve the suppression of osteoblast function. To elucidate the influence of mechanical unloading on osteoblasts, a rotating wall vessel (RWV) was employed as a ground based model of simulated microgravity. Mouse MC3T3-E1 osteoblasts were grown on microcarrier beads for 14 days and then placed in the RWV for 24 h. Consistent with decreased bone formation during actual spaceflight conditions, alkaline phosphatase and osteocalcin expression were decreased by 80 and 50%, respectively. In addition, runx2 expression and AP-1 transactivation, key regulators of osteoblast differentiation and bone formation, were reduced by more than 60%. This finding suggests that simulated microgravity could promote dedifferentiation and/or transdifferentiation to alternative cell types; however, markers of adipocyte, chondrocyte, and myocyte lineages were not induced by RWV exposure. Taken together, our results indicate that simulated microgravity may suppress osteoblast differentiation through decreased runx2 and AP-1 activities.

Changes in Runx2/Cbfa1 expression and activity during osteoblastic differentiation of human bone marrow stromal cells

Runx2/Cbfa1 has been identified as a "master gene" controlling osteoblast differentiation. However, its role in inducing the osteoblast phenotype has been characterized primarily in rodent systems. Thus, we examined Runx2/Cbfa1 messenger RNA, protein, and activity levels during osteoblastic differentiation of human bone marrow stromal (BMSC) cells. Semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that the expression of alkaline phosphatase and osteocalcin mRNAs increased in a time-dependent manner with the development of the osteoblast phenotype by these cells (hMS2-15). Type II Runx2/Cbfa1 messenger RNA was found to be constitutively expressed in hMS2-15 cells and not altered during differentiation; there was no detectable expression of the type I Runx2/Cbfa1 transcript. Interestingly, despite the absence of any change in Runx2/Cbfa1 messenger RNA levels during osteoblastic differentiation of these cells, the activity of Runx2/Cbfa1, as assessed by binding to the osteoblast-specific cis-acting element 2 (OSE2), increased markedly at all time-points examined, with the highest activity level seen at day 7. Similar results were observed in primary cultures of less differentiated human marrow-derived mesenchymal stem cells. Immunoprecipitation and Western blot analysis revealed that whereas there was no increase in Runx2/Cbfa1 protein levels with differentiation in hMS2-15 cells, there was an increase in Runx2/Cbfa1 phosphorylation. Thus, in contrast to rodent systems where osteoblast differentiation is associated with increased synthesis of Runx2/Cbfa1, we find that in human BMSC, osteoblastic differentiation is associated primarily with increases in Runx2/Cbfa1 activity, without a change in messenger RNA or protein levels. Our findings also show that the increase in Runx2/Cbfa1 activity occurs through a posttranslational mechanism involving phosphorylation of key residues.

Transcriptional coactivation of bone-specific transcription factor Cbfa1 by TAZ

Core-binding factor 1 (Cbfa1; also called Runx2) is a transcription factor belonging to the Runt family of transcription factors that binds to an osteoblast-specific cis-acting element (OSE2) activating the expression of osteocalcin, an osteoblast-specific gene. Using the yeast two-hybrid system, we identified a transcriptional coactivator, TAZ (transcriptional coactivator with PDZ-binding motif), that binds to Cbfa1. A functional relationship between Cbfa1 and TAZ is demonstrated by the coimmunoprecipitation of TAZ by Cbfa1 and by the fact that TAZ induces a dose-dependent increase in the activity of osteocalcin promoter-luciferase constructs by Cbfa1. A dominant-negative construct of TAZ in which the coactivation domains have been deleted reduces osteocalcin gene expression down to basal levels. NIH 3T3, MC 3T3, and ROS 17/2.8 cells showed the expected nuclear localization of Cbfa1, whereas TAZ was distributed throughout the cytoplasm with some nuclear localization when transfected with either Cbfa1 or TAZ. Upon cotransfection by both Cbfa1 and TAZ, the transfected TAZ shows predominant nuclear localization. The dominant-negative construct of TAZ shows minimal nuclear localization upon cotransfection with Cbfa1. These data indicate that TAZ is a transcription coactivator for Cbfa1 and may be involved in the regulation of osteoblast differentiation.

The protein kinase C pathway plays a central role in the fibroblast growth factor-stimulated expression and transactivation activity of Runx2

Fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling induces the expression of Runx2, a key transcription factor in osteoblast differentiation, but little is known about the molecular signaling mechanisms that mediate this. Here we examined the role of the protein kinase C (PKC) pathway in regulating Runx2 gene expression and its transactivation function. Treatment with FGF2 or FGF4, or transfection with a vector expressing a mutant FGFR2 that is constitutively activated in the absence of ligand, strongly stimulates Runx2 expression. Electrophoretic mobility shift assays also showed that FGF2 treatment increases the specific binding of Runx2 to the cognate response element in the osteocalcin gene promoter. Blocking PKC completely inhibited FGF2-induced Runx2 expression, whereas mitogen-activate protein kinase inhibitors had no effect. The FGF/FGFR-stimulated 6xOSE2 promoter activity was also blocked by inhibiting PKC, as was the FGF2 stimulation of the DNA-binding activity of Runx2. Experiments with PKC isoform-specific inhibitors and dominant negative isoforms of PKC indicate that PKCdelta is one of key isoforms involved in the FGF2-stimulated Runx2 expression. In addition, experiments with Runx2-knockout cells showed that, although the PKC pathway largely regulates FGF2-stimulated Runx2 activity by up-regulating Runx2 expression, it also modifies Runx2 protein post-translationally and thereby increases its transcriptional activity. Thus, we show for the first time that FGF/FGFR signaling stimulates the DNA-binding and transcriptional activities of Runx2 as well as its expression, and these are largely regulated by the PKC pathway.

Increased production of IL-7 uncouples bone formation from bone resorption during estrogen deficiency

Postmenopausal bone loss stems from the inability of osteoblastic activity to match the increase in osteoclastic bone resorption induced by estrogen deficiency. However, the mechanism that uncouples osteoblast from osteoclast activities remains unexplained. We show that ovariectomy enhances the production of the osteoclastogenic cytokine IL-7, and that its neutralization in vivo prevents ovariectomy-induced bone loss. Surprisingly, serum osteocalcin levels, a biochemical marker of bone formation, suggested that the bone-sparing effects of IL-7 neutralization were due not only to inhibition of bone resorption, but also to stimulation of bone formation. Consistent with these data, addition of IL-7 to neonatal calvarial organ cultures blocked new bone formation, and injection of IL-7 into mice in vivo inhibited bone formation as measured by calcein incorporation into long bones. The antianabolic effects of IL-7 were consistent with an observed downregulation of the osteoblast-specific transcription factor core-binding factor alpha1/Runx2. Thus, because it targets both the osteoclast and the osteoblast pathways, IL-7 is central to the altered bone turnover characteristic of estrogen deficiency.

Bone morphogenetic protein-2 inhibits differentiation and mineralization of cementoblasts in vitro

As an approach for improving the outcome and predictability of periodontal regenerative therapies, we have focused on determining the responses of cells within the local environment to putative regenerative factors. This study examined the effects of bone morphogenetic protein-2 (BMP-2) on murine cementoblasts in vitro. Northern blot analysis indicated that BMP-2 decreased mRNA levels of bone sialoprotein and type I collagen dose-dependently (10-300 ng/mL). At low doses, up to 100 ng/mL, BMP-2 had no effect on transcripts for osteocalcin and osteopontin, whereas at 300 ng/mL, BMP-2 greatly increased expression of these two genes. BMP-2 also inhibited cementoblast-mediated mineral nodule formation in a dose-dependent manner (inhibition was noted at 10 ng/mL). Noggin reversed the effects of BMP-2 on gene expression and on mineralization. These findings reflect the diverse responses of periodontal cells to BMP-2 and highlight the need to consider the complexity of factors involved in designing predictable regenerative therapies.

A short pulse of mechanical force induces gene expression and growth in MC3T3-E1 osteoblasts via an ERK 1/2 pathway

Physiological mechanical loading is crucial for maintenance of bone integrity and architecture. We have calculated the strain caused by gravity stress on osteoblasts and found that 4-30g corresponds to physiological levels of 40-300 microstrain. Short-term gravity loading (15 minutes) induced a 15-fold increase in expression of growth-related immediate early gene c-fos, a 5-fold increase in egr-1, and a 3-fold increase in autocrine bFGF. The non-growth-related genes EP-1, TGF-beta, and 18s were unaffected by gravity loading. Short-term physiological loading induced extracellular signal-regulated kinase (ERK 1/2) phosphorylation in a dose-dependent manner with maximum phosphorylation saturating at mechanical loading levels of 12g (p < 0.001) with no effect on total ERK. The phosphorylation of focal adhesion kinase (FAK) was unaffected by mechanical force. g-Loading did not activate P38 MAPK or c-jun N-terminal kinase (JNK). Additionally, a gravity pulse resulted in the localization of phosphorylated ERK 1/2 to the nucleus; this did not occur in unloaded cells. The induction of c-fos was inhibited 74% by the MEK1/2 inhibitor U0126 (p < 0.001) but was not affected by MEK1 or p38 MAPK-specific inhibitors. The long-term consequence of a single 15-minute gravity pulse was a 64% increase in cell growth (p < 0.001). U0126 significantly inhibited gravity-induced growth by 50% (p < 0.001). These studies suggest that short periods of physiological mechanical stress induce immediate early gene expression and growth in MC3T3-E1 osteoblasts primarily through an ERK 1/2-mediated pathway.

Increased production of IL-7 uncouples bone formation from bone resorption during estrogen deficiency

Postmenopausal bone loss stems from the inability of osteoblastic activity to match the increase in osteoclastic bone resorption induced by estrogen deficiency. However, the mechanism that uncouples osteoblast from osteoclast activities remains unexplained. We show that ovariectomy enhances the production of the osteoclastogenic cytokine IL-7, and that its neutralization in vivo prevents ovariectomy-induced bone loss. Surprisingly, serum osteocalcin levels, a biochemical marker of bone formation, suggested that the bone-sparing effects of IL-7 neutralization were due not only to inhibition of bone resorption, but also to stimulation of bone formation. Consistent with these data, addition of IL-7 to neonatal calvarial organ cultures blocked new bone formation, and injection of IL-7 into mice in vivo inhibited bone formation as measured by calcein incorporation into long bones. The antianabolic effects of IL-7 were consistent with an observed downregulation of the osteoblast-specific transcription factor core-binding factor alpha1/Runx2. Thus, because it targets both the osteoclast and the osteoblast pathways, IL-7 is central to the altered bone turnover characteristic of estrogen deficiency.