Bone morphogenetic proteins, extracellular matrix, and mitogen-activated protein kinase signaling pathways are required for osteoblast-specific gene expression and differentiation in MC3T3-E1 cells

Noggin inhibits chondrogenic but not osteogenic differentiation in mesodermal stem cell line C1 and skeletal cells

Osteoblasts and chondroblasts are derived from common mesenchymal progenitors. Although bone morphogenetic protein induces mesenchymal differentiation into both osteogenic and chodrogenic lineage cells in vitro, its inhibitor, Noggin, is expressed exclusively during chondrogenic but not osteogenic differentiation in an embryonal carcinoma-derived mesodermal cell line, C1. We hypothesized that Noggin may regulate cell differentiation in a lineage-specific manner. To test this hypothesis, Noggin was overexpressed using recombinant adenovirus (Ad/Noggin) in mesodermal C1 cells to examine whether Noggin specifically inhibits chondrogenic differentiation. Noggin overexpression by recombinant adenovirus infection reduced Sox9, patched, Ihh, and type II, X, and XI collagen mRNA expression levels in C1 cell aggregates that were induced to differentiate into chondrocyte lineage by culturing in differentiation medium. In contrast, Noggin overexpression did not affect osteogenic differentiation in C1 cells because osteoblast phenotypic markers such as osteocalcin and alkaline phosphatase mRNA levels were not altered. We further examined whether Noggin also differentially affects chondrogenesis and osteogenesis in limb development by using organ cultures of long bone. Ad/Noggin infection into 15.5 d post conception limb skeletal rudiments that were cultured on filter membrane in vitro or on the chorioallantoic membranes in ovo inhibited the levels of chondrogenesis, which were evaluated based on alcian blue staining. These results suggest that Noggin specifically blocks chondrogenic differentiation, rather than osteogenic differentiation, in mesodermal stem cell line C1 and skeletal cells.

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.

Impact of the mitogen-activated protein kinase pathway on parathyroid hormone-related protein actions in osteoblasts

Parathyroid hormone-related protein (PTHrP) regulates proliferation and differentiation of osteoblastic cells via binding to the parathyroid hormone receptor (PTH-1R). The cAMP-dependent protein kinase A pathway governs the majority of these effects, but recent evidence also implicates the MAPK pathway. MC3T3-E1 subclone 4 cells (MC4) were treated with the MAPK inhibitor U0126 and PTHrP. In differentiated MC4 cells, osteocalcin and bone sialoprotein gene expression were both down-regulated by PTHrP and also by inhibition of the MAPK pathway. PTHrP-mediated down-regulation of PTH-1R mRNA and up-regulation of c-fos mRNA were MAPK-independent, whereas PTHrP stimulation of fra-2 and interleukin-6 (IL-6) mRNA was MAPK-dependent. Luciferase promoter assays revealed that regulation of IL-6 involved the cAMP-dependent protein kinase A and MAPK pathways with a potential minor role of the protein kinase C pathway, and a promoter region containing an activator protein-1 site was necessary for PTHrP-induced IL-6 gene transcription. An alternative pathway, through cAMP/Epac/Rap1/MAPK, mediated ERK phosphorylation but was not sufficient for IL-6 promoter activation. Phosphorylation of the transcription factor CREB was also necessary but not sufficient for PTHrP-mediated IL-6 promoter activity. Most interesting, a bidirectional effect was found with PTHrP increasing phosphorylated ERK in undifferentiated MC4 cells but decreasing phosphorylated ERK in differentiated cells. These data indicate that inactivation of the MAPK pathway shows differential regulation of PTHrP-stimulated activator protein-1 members, blocks PTHrP-stimulated IL-6, and synergistically down-regulates certain osteoblastic markers associated with differentiation. These novel findings indicate that the MAPK pathway plays a selective but important role in the actions of PTHrP.

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.

Changes in gene expression in response to mechanical strain in human scleral fibroblasts

Scleral fibroblasts are involved in scleral remodeling during axial elongation in myopia. Mechanical load is a potent stimulator of gene expression. This study seeks to identify changes in gene expression of scleral fibroblasts in response to mechanical load and speculate on possible mechanisms of scleral remodeling in the development of myopia. Human scleral fibroblasts (HSFs) were mechanically stretched for 30 min and 24 hr. A gene microarray analysis was used to measure changes in gene expression. A total of 237 genes revealed differential and significant changes in expression (P<0.01) after 30 min of stretching. Of these, 28 unexpressed genes began to be expressed (turned on), while 31 expressed genes were no longer expressed (turned off). After 24 hr, 308 genes showed reproducible changes in expression (P<0.01), while 29 genes were turned on and 17 genes were turned off. After 30 min, 25 genes showed at least a threefold change in expression. These included genes for cell receptors, protein kinases, cell growth/differentiation factors, extracellular matrix (ECM) proteins, lipid metabolism, protein metabolism, transcription factors, binding proteins and water channels. After 24 hr, 21 genes showed at least a threefold change in expression. These included genes for cell receptors, protein kinases, cell growth/differentiation factors, lipid metabolism, ECM proteins, transcription factors, and carbohydrate metabolism. RT-PCR and Southern blotting confirmed the changes in expression of selected genes. In this study we identified a large number of early and late mechanical response genes in HSFs. These changes in gene expression will provide potential candidate genes that might be involved in scleral remodeling during axial elongation in myopia.

Adhesion to Vitronectin and Collagen I Promotes Osteogenic Differentiation of Human Mesenchymal Stem Cells

The mechanisms controlling human mesenchymal stem cells (hMSC) differentiation are not entirely understood. We hypothesized that the contact with extracellular matrix (ECM) proteins normally found in bone marrow would promote osteogenic differentiation of hMSC in vitro. To test this hypothesis, we cultured hMSC on purified ECM proteins in the presence or absence of soluble osteogenic supplements, and assayed for the presence of well-established differentiation markers (production of mineralized matrix, osteopontin, osteocalcin, collagen I, and alkaline phosphatase expression) over a 16-day time course. We found that hMSC adhere to ECM proteins with varying affinity (fibronectin>collagen I≥collagen IV≥vitronectin>laminin-1) and through distinct integrin receptors. Importantly, the greatest osteogenic differentiation occurred in cells plated on vitronectin and collagen I and almost no differentiation took place on fibronectin or uncoated plates. We conclude that the contact with vitronectin and collagen I promotes the osteogenic differentiation of hMSC, and that ECM contact alone may be sufficient to induce differentiation in these cells.

New targets for therapy in prostate cancer: modulation of stromal-epithelial interactions

Changes in genomic and phenotypic expression of progressing prostate tumors and their stroma occur in a dynamic fashion based on bidirectional signaling from stromal-epithelial interactions. These interactions may underlie the ability of prostate cancer cells to survive and proliferate in the prostate and bone. By investigating the phenotypic and genotypic changes of stromal cells adjacent to cancer cells and the reciprocal changes of cancer cells, novel molecular markers may be developed to diagnose cancer earlier before pathologic appearance of cancer cells at the primary site. Attacking epithelial and stromal elements together is a unique approach to both localized and metastatic prostate cancer therapy. Co-targeting both tumor cells and stroma requires identifying a reliable tumor and tissue-specific cis-DNA element, such as osteocalcin (OC) promoter. OC expression is elevated in prostate tumor cells and in prostate and bone stromal cells interdigitating with both localized and metastatic prostate epithelium. We have previously designed an adenovirus-based therapeutic gene vehicle and demonstrated that a replication-competent adenoviral vector (Ad vector) is highly efficient in blocking the growth of cancer cells in culture, including cells without androgen receptor as well as cells that do or do not make prostate-specific antigen. In vivo, intravenous administration of an Ad-OC vector was effective against preexisting human prostate cancer subcutaneous and bone xenografts. The addition of vitamin D(3) enhanced further viral replication at target sites. Co-targeting tumor cells and stroma using systemic Ad vector is a viable and promising option for treatment of both localized and metastatic prostate cancer.

Deletion mutants of BMP folding variants act as BMP antagonists and are efficient inhibitors for heterotopic ossification

Heterotopic ossification is a frequent complication in patients who have suffered head and neck traumas or have undergone total hip replacement. In this report, stable folding variants of the natural occurring osteoinductive BMPs were shown to act as inhibitors for heterotopic ossification. The most effective BMP folding variant construct performed even better than the natural occurring BMP antagonist Noggin because it also inhibited calcium deposition of pre-osteoblastic cells.

INTRODUCTION

Signal transduction through receptor and ligand binding depends on the proper folding of all partners, especially when it involves the formation of a heterotetramer. In the case, the receptor binding of the ligand can be uncoupled from signal transduction, and folding variants of a ligand can be developed into antagonists of the natural bioactivity of the ligand. Here we present a deletion mutant of a bone morphogenetic protein (BMP) folding variant capable of inhibiting the bone-inducing action of natural occurring BMPs.

MATERIALS AND METHODS

Deletion mutants and site-directed mutants of BMP folding variants were generated and tested for their ability to reduce alkaline phosphatase activity and mineralization in a pre-osteoblastic cell line. In vivo activity of the optimized folding variant was determined in a heterotopic ossification model in rodents and in two Xenopus laevis model systems. Biosensor interaction analysis was used to determine the affinity of the optimized BMP folding variant to the extracellular domain of BMP receptors.

RESULTS

In vitro and in vivo tests in rodents revealed that the structural elements of the wrist epitope combined with finger 2 and a positive charge proximal to the tip of this finger are sufficient to induce osteoinhibition with deletion mutants and folding variants of mature BMP-4. The inhibitor designed to suppress heterotopic ossification showed BMP antagonist activity in embryos and animal caps of X. laevis. Binding studies of the inhibitor to ectodomains of type I and type II BMP receptors revealed a concentration-dependent binding, especially to the high-affinity BMP receptor.

CONCLUSIONS

Deletion mutants of BMP folding variants are a new form of BMP antagonists and act through competition with osteoinductive BMP for BMP receptor binding. The excellent in vivo performance of the optimized folding variant is because of its ability to block signaling of endogenous BMPs deposited in the extracellular matrix even more effectively than the natural occurring BMP antagonist Noggin.

Parathyroid hormone induction of the osteocalcin gene. Requirement for an osteoblast-specific element 1 sequence in the promoter and involvement of multiple-signaling pathways

Parathyroid hormone (PTH) is an important peptide hormone regulator of bone formation and osteoblast activity. However, its mechanism of action in bone cells is largely unknown. This study examined the effect of PTH on mouse osteocalcin gene expression in MC3T3-E1 preosteoblastic cells and primary cultures of bone marrow stromal cells. PTH increased the levels of osteocalcin mRNA 4-5-fold in both cell types. PTH also stimulated transcriptional activity of a 1.3-kb fragment of the mouse osteocalcin gene 2 (mOG2) promoter. Inhibitor studies revealed a requirement for protein kinase A, protein kinase C, and mitogen-activated protein kinase pathways in the PTH response. Deletion of the mOG2 promoter sequence from -1316 to -116 caused no loss in PTH responsiveness whereas deletion from -116 to -34 completely prevented PTH stimulation. Interestingly, this promoter region does not contain the RUNX2 binding site shown to be necessary for PTH responsiveness in other systems. Nuclear extracts from PTH-treated MC3T3-E1 cells exhibited increased binding to OSE1, a previously described osteoblast-specific enhancer in the mOG2 promoter. Furthermore, mutation of OSE1 in DNA transfection assays established the requirement for this element in the PTH response. Collectively, these studies establish that actions of PTH on the osteocalcin gene are mediated by multiple signaling pathways and require OSE1 and associated nuclear proteins.

Clodronate stimulates osteoblast differentiation in ST2 and MC3T3-E1 cells and rat organ cultures

We investigated the direct effects of various bisphosphonates on osteoblasts. At 10(-5) M, clodronate increased alkaline phosphatase activity in cultured MC3T3-E1 (osteoblast-like line) and ST2 (pluripotent mesenchymal line) cells. Etidronate significantly increased alkaline phosphatase activity at 10(-5) M only in MC3T3-E1 cells. These effects were due to an increase in alkaline phosphatase-positive cell numbers, and the differentiation-enhanced cells were capable of mineralization (von Kossa stain). Other bisphosphonates (pamidronate, alendronate, and incadronate) did not increase alkaline phosphatase activity in either cell line. In cultured rat calvariae, clodronate stimulated the expression of genes for alkaline phosphatase and osteocalcin (osteoblast-differentiation markers), but decreased the expression of the gene for tartrate-resistant acid phosphatase (osteoclast marker). Clodronate, etidronate, and incadronate inhibited protein Tyr phosphatase and Ser/Thr phosphatase activities in MC3T3-E1 cells. These data suggest that clodronate acts directly on mesenchymal cells to enhance osteoblast differentiation, and this effect may be partly expressed through inhibition of protein Tyr phosphatase and/or Ser/Thr phosphatase activity.

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.

Ammonium detoxification performed by porcine hepatocyte spheroids in a bioartificial liver for pediatric use: preliminary report

Bioartificial liver devices are alternative therapies for patients suffering from acute hepatic failure or metabolic defects. Here, we show a bioartificial device, developed with a cartridge used for pediatric hemofiltration and spheroids of porcine hepatocytes housed in the extracapillary space of the cartridge. The cartridge was attached to a robotic arm that supplied a continuous, oscillatory movement. It was connected through the capillary circulation to a neonatal membrane oxygenator contain-ing human blood supplemented with ammonium and diazepam. A decrease in ammonium concentration was observed, reaching an almost 70% decrease upon 9 h of operation. In addition, urea was detected and diazepam metabolism proved from the fourth hour of operation. It is worth mentioning that the system described was assembled with commercially available components for current clinical use. The setup may be done in a short period, thus eliminating long-term culture times and the need for cell anchoring to matrices.

Bone morphogenetic protein-2 restores mineralization in glucocorticoid-inhibited MC3T3-E1 osteoblast cultures

The anti-glucocorticoid potential of BMP-2 in osteoblasts was tested in MC3T3-E1 cells using dexamethasone (1 microM) and rhBMP-2 (10 or 100 ng/ml). rhBMP-2 restored mineralization but not condensation or collagen accumulation. These results demonstrate the potential and limitations of BMPs in counteracting glucocorticoids.

INTRODUCTION

Pharmacologic glucocorticoids (GCs) inhibit osteoblast function and induce osteoporosis. Bone morphogenetic proteins (BMPs) stimulate osteoblast differentiation and bone formation. Here we tested the anti-glucocorticoid potential of BMP-2 in cultured osteoblasts.

MATERIALS AND METHODS

MC3T3-E1 cells were treated with dexamethasone (DEX; 1 microM) and/or recombinant human BMP-2 (rhBMP-2; 10 or 100 ng/ml). Culture progression was characterized by cell cycle profiling, biochemical assays for DNA, alkaline phosphatase (ALP), collagen, and calcium, and by reverse transcriptase-polymerase chain reaction (RT-PCR) of osteoblast phenotypic mRNAs. Mineralization was characterized by Alizarin red and von Kossa staining and by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD).

RESULTS

DEX inhibited differentiation-related cell cycle, nodule formation, collagen accumulation, osteocalcin, and BMP-2 gene expression as well as mineralization. Replenishment of GC-inhibited cultures with 10 or 100 ng/ml rhBMP-2 dramatically rescued mineral deposition. The rhBMP-2-rescued mineral was bone-like apatite nearly identical to the mineral of control cultures. The rhBMP-2 rescue was associated with increased mRNA levels for alpha1(I) collagen, osteocalcin, and Cbfa1 types I and II, as well as ALP activity. In contrast, rhBMP-2 did not rescue the GC-inhibited differentiation-related cell cycle, nodule formation, or collagen accumulation. When administered alone, rhBMP-2 also increased the mRNA levels for alpha1(I) collagen, osteocalcin, and Cbfa1 types I and II, as well as ALP activity. However, treatment with rhBMP-2 alone inhibited cell cycle progression, nodule formation, and collagen accumulation. Surprisingly, in contrast to its rescue of mineralization in DEX-treated cultures, rhBMP-2 inhibited mineralization in the absence of DEX. In parallel to its bimodal effect on mineralization, rhBMP-2 stimulated endogenous BMP-2 mRNA in the presence of DEX, but inhibited endogenous BMP-2 mRNA in the absence of DEX.

CONCLUSIONS

Suppression of BMP-2 gene expression plays a pivotal role in GC inhibition of osteoblast differentiation. However, the inability of rhBMP-2 to rescue the entire osteoblast phenotype suggests BMP-2-independent inhibitory effects of CCs. BMP-2 exerts both positive and negative effects on osteoblasts, possibly depending on the differentiation stage and/or the existing BMP signaling.

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.

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.

Inhibition of phosphatidylinositol 3-kinase and p70S6 kinase blocks osteogenic protein-1 induction of alkaline phosphatase activity in fetal rat calvaria cells

Published studies reveal that Osteogenic Protein-1 (OP-1) and insulin-like growth factor-I (IGF-I) synergistically stimulate alkaline phosphatase (AP) activity and bone nodule formation in fetal rat calvaria (FRC) cells. In the present study, we examined whether there are interactions between the signal transduction pathways activated by these two growth factors. OP-1 did not significantly affect the levels of IRS-1, IRS-2, the p85alpha subunit of phosphatidylinositol 3-kinase (PI 3-kinase) or the extracellular signal-regulated kinase (ERK)-2, but stimulated ERK-1 protein by twofold. OP-1 also induced phosphorylation of ERK-1 and -2, but not of Akt/protein kinase B (PKB), a protein kinase that is downstream of PI 3-kinase. By comparison, IGF-I increased the levels of the phosphorylated forms of ERK-1 and -2, and Akt/PKB. Inhibition of ERK activation by PD98059 did not significantly alter the stimulation of AP activity by OP-1 or OP-1 in combination with IGF-I. In contrast, inhibition of PI 3-kinase activity by LY294002 blocked the induction of AP activity by OP-1 and OP-1 plus IGF-I. Treatment of cells with rapamycin, an inhibitor of the mammalian target of mTOR, resulted in a 47% and a 53% decrease in the AP activity induced by OP-1 alone and by OP-1 plus IGF-I, respectively. These studies suggest that PI 3-kinase and mTOR contribute to the induction of AP activity by OP-1 and the synergistic effect of OP-1 and IGF-I on AP activity in FRC cells.

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.

Intranuclear trafficking of transcription factors: Requirements for vitamin D-mediated biological control of gene expression

The architecturally associated subnuclear organization of nucleic acids and cognate regulatory factors suggest functional interrelationships between nuclear structure and gene expression. Mechanisms that contribute to the spatial distribution of transcription factors within the three-dimensional context of nuclear architecture control the sorting of regulatory information as well as the assembly and activities of sites within the nucleus that support gene expression. Vitamin D control of gene expression serves as a paradigm for experimentally addressing mechanisms that govern the intranuclear targeting of regulatory factors to nuclear domains where transcription of developmental and tissue-specific genes occur. We will present an overview of molecular, cellular, genetic, and biochemical approaches that provide insight into the trafficking of regulatory factors that mediate vitamin D control of gene expression to transcriptionally active subnuclear sites. Examples will be presented that suggest modifications in the intranuclear targeting of transcription factors abrogate competency for vitamin D control of skeletal gene expression during development and fidelity of gene expression in tumor cells.

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.

Estrogens activate bone morphogenetic protein-2 gene transcription in mouse mesenchymal stem cells

Estrogens exert their physiological effects on target tissues by interacting with the estrogen receptors, ERalpha and ERbeta. Estrogen replacement is one the most common and effective strategies used to prevent osteoporosis in postmenopausal women. Whereas it was thought that estrogens work exclusively by inhibiting bone resorption, our previous results show that 17beta-estradiol (E2) increases mouse bone morphogenetic protein (BMP)-2 mRNA, suggesting that estrogens may also enhance bone formation. In this study, we used quantitative real-time RT-PCR analysis to demonstrate that estrogens increase BMP-2 mRNA in mouse mesenchymal stem cells. The selective ER modulators, tamoxifen, raloxifene, and ICI-182,780 (ICI), failed to enhance BMP-2 mRNA, whereas ICI inhibited E2 stimulation of expression. To investigate if estrogens increase BMP-2 expression by transcriptional mechanisms and if the response is mediated by ERalpha and/or ERbeta, we studied the effects of estrogens on BMP-2 promoter activity in transient transfected C3H10T1/2 cells. E2 produced a dose-dependent induction of the mouse -2712 BMP-2 promoter activity in cells cotransfected with ERalpha and ERbeta. At a dose of 10 nM E2, ERalpha induced mouse BMP-2 promoter activity 9-fold, whereas a 3-fold increase was observed in cells cotransfected with ERbeta. Tamoxifen and raloxifene were weak activators of the mouse BMP-2 promoter via ERalpha, but not via ERbeta. ICI blocked the activation of BMP-2 promoter activity by E2 acting via both ERalpha and ERbeta, indicating that mouse BMP-2 promoter activation is ER dependent. In contrast to E2 and selective ER modulators, the phytoestrogen, genistein was more effective at activating the mouse BMP-2 promoter with ERbeta, compared with ERalpha. Using a deletion series of the BMP-2 promoter, we determined that AP-1 or Sp1 sites are not required for E2 activation. A mutation in a sequence at -415 to -402 (5'-GGGCCActcTGACCC-3') that resembles the classical estrogen-responsive element abolished the activation of the BMP-2 promoter in response to E2. Our studies demonstrate that E2 activation of mouse BMP-2 gene transcription requires ERalpha or ERbeta acting via a variant estrogen-responsive element binding site in the promoter, with ERalpha being the more efficacious regulator. Estrogenic compounds may enhance bone formation by increasing the transcription of the BMP-2 gene.

Continuous inhibition of MAPK signaling promotes the early osteoblastic differentiation and mineralization of the extracellular matrix

We screened the small molecule compounds that stimulate osteogenesis by themselves or promote bone morphogenetic protein (BMP)-induced bone formation. We found that a specific inhibitor for MAPK/extracellular signal-regulated kinase kinase (MEK)-1, promoted the early osteoblastic differentiation and mineralization of extracellular matrix (ECM) in C2Cl2 pluripotent mesenchymal cells treated with recombinant human BMP-2 (rhBMP-2) and MC3T3-E1 preosteoblastic cells. ALP activity was synergistically increased by the treatment with a specific MEK-1 inhibitor PD98059 and rhBMP-2 in both cell lines. Twenty-five micromolar PD98059 promoted mineralization of ECM in rhBMP-2-treated C2Cl2 cells and MC3T3-E1 cells. In contrast, PD98059 reduced osteocalcin (OCN) secretion and its transcriptional level in rhBMP-2-treated C2Cl2 cells but increased its secretion and mRNA level in MC3T3-E1 cells. Stable expression of a dominant-negative MEK-1 mutant in C2Cl2 cells represented high ALP activity and low osteocalcin production in the presence of rhBMP-2, while a constitutively active mutant of MEK-1 attenuated both of them. Together, our results indicated that BMP-2-induced mineralization of ECM of pluripotent mesenchymal stem cells and preosteoblastic cells could be controlled by a fine tuning of the MAPK signaling pathway. Further, MEK-1 inhibitors would be useful for the promotion of bone formation, for instance, the treatments for delayed fracture healing or advance of localized osteoporotic change after fracture healing.

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

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