Assessment of gene regulation by bone morphogenetic protein 2 in human marrow stromal cells using gene array technology

Cooperation between TGF-beta and Wnt pathways during chondrocyte and adipocyte differentiation of human marrow stromal cells

Human marrow stromal cells have the potential to differentiate to chondrocytes or adipocytes. We show interactions between TGF-beta and Wnt signaling pathways during stimulation of chondrogenesis and inhibition of adipogenesis. Combining these signals may be useful in marrow stromal cell therapies.

INTRODUCTION

Human bone marrow stromal cells (hMSCs) have the potential to differentiate to lineages of mesenchymal tissues, including cartilage, fat, bone, tendon, and muscle. Agents like transforming growth factor (TGF)-beta promote chondrocyte differentiation at the expense of adipocyte differentiation. In other processes, TGF-beta and Wnt/wingless signaling pathways play major roles in controling certain developmental events and activation of specific target genes. We tested whether these pathways interact during differentiation of chondrocytes and adipocytes in human marrow stromal cells.

MATERIALS AND METHODS

Both a line of human marrow stromal cells (KM101) and freshly isolated hMSCs were studied. Reverse transcriptase-polymerase chain reaction (RT-PCR), Western blot, and macroarrays were used for analysis of the modulation of TGF-beta1 on Wnt signaling-associated genes, chondrocyte differentiation genes, and TGFbeta/bone morphogenetic protein (BMP) signaling-associated genes in KM101 cells. Early passage hMSCs obtained from 42- and 58-year-old women were used for the effects of TGF-beta and/or Wnt (mimicked by LiCl) signals on chondrocyte and adipocyte differentiation in two-dimensional (2-D) cultures, 3-D pellet cultures, and collagen sponges.

RESULTS

As indicated by macroarray, RT-PCR, and Western blot, TGF-beta activated genes in the TGF-beta/Smad pathway, upregulated Wnt2, Wnt4, Wnt5a, Wnt7a, Wnt10a, and Wnt co-receptor LRP5, and increased nuclear accumulation and stability of beta-catenin in KM101 cells. TGF-beta upregulated chondrocyte gene expression in KM101 cells and also stimulated chondrocyte differentiation and inhibited adipocyte differentiation in hMSCs, synergistically with Wnt signal. Finally, hMSCs cultured in 3-D collagen sponges were stimulated by TGF-beta1 to express aggrecan and collagen type II mRNA, whereas expression of lipoprotein lipase was inhibited.

CONCLUSIONS

In summary, TGF-beta stimulated chondrocyte differentiation and inhibited adipocyte differentiation of hMSCs in vitro. The activation of both TGF-beta and Wnt signal pathways by TGF-beta, and synergy between TGF-beta and Wnt signals, supports the view that Wnt-mediated signaling is one of the mechanisms of TGF-beta's effects on chondrocyte and adipocyte differentiation of hMSCs.

Transcriptional regulation of BMP4 synexpression in transgenic Xenopus

Synexpression groups are genetic modules composed of genes that share both a complex expression pattern and the biological process in which they function. Here we investigate the regulation of BMP4 synexpression by studying the enhancers of bambi, smad7 and vent2 in Xenopus. We find that a BMP4 synexpression promoter module is compact and (i) requires direct BMP responsiveness through Smad and Smad-cofactor binding motifs, (ii) may contain an evolutionary conserved BMP-responsive element, bre7 (TGGCGCC), that is crucial for expression of bambi and smad7 and is highly prognostic for novel BMP-responsive enhancers (BREs); and (iii) requires a narrow window of BMP inducibility, because minor enhancement or reduction of BMP responsiveness abolishes synexpression. Furthermore, we used a bioinformatic model to predict in silico 13 novel BREs, and tested five of them that were found in the id1-4 genes. The results highlight that in vivo analysis is required to reveal the physiological, spatio-temporal regulation of BMP-responsive genes.

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.

Adipogenic differentiation of human adult stem cells from bone marrow stroma (MSCs)

We assayed gene expressions during adipogenesis of human MSCs. Microarray assays demonstrated time-dependent increases in expression of 67 genes, including 2 genes for transcription factors that were not previously shown to be expressed during adipogenesis. INTRODUCTION Increased numbers of bone marrow adipocytes have been observed in patients with osteoporosis and aplastic anemia, but the pathological mechanisms remain unknown. Recently, microarray assays for mRNAs were used to follow adipogenic differentiation of the preadipocytic cell line, 3T3-L1, but adipogenic differentiation has not been examined in primary cells from bone marrow. Here we defined the sequence of gene expression during the adipogenesis ex vivo of human cells from bone marrow referred to as either mesenchymal stem cells or marrow stromal cells (MSCs).

MATERIALS AND METHODS

MSCs were plated at extremely low densities to generate single-cell derived colonies, and adipogenic differentiation of the colonies assayed by accumulation of fat vacuoles, time-lapse photomicroscopy, microarrays, and reverse transcriptase-polymerase chain reaction (RT-PCR) assays. RESULTS AND CONCLUSIONS About 30% of the colonies differentiated to adipocytes in 14 days and about 60% in 21 days. Cell proliferation was inhibited by approximately 50% in adipogenic medium. The differentiation occurred primarily at the center of the colonies, and a few adipocytes that formed near the periphery migrated toward the centers. RT-PCR assays demonstrated that the differentiation was accompanied by increases in a series of genes previously shown to increase with adipogenic differentiation: peroxisome proliferator activated receptor gamma, CCAAT enhancer-binding protein alpha, acylCoA synthetase, lipoprotein lipase, and fatty acid binding protein 4. We also followed differentiation with microarray assays. Sixty-seven genes increased more than 10-fold at day 1 and 20-fold at day 7, 14, or 21. Many of the genes identified were previously shown to be expressed during adipocytic differentiation. However, others, such as zinc finger E-box binding protein and zinc finger protein 145, were not. This study should serve as a basis for future study to clarify the mechanisms of adipocyte differentiation of MSCs.

Conservation of Sox4 gene structure and expression during chicken embryogenesis

While mutations in Sox4, a member of the SRY-like HMG box gene family, have been associated with a variety of human disorders and embryonic defects in the mouse, the structure and developmental expression of Sox4 in the avian embryo has not been described. We have isolated and characterized the chicken Sox4 gene. The chicken Sox4 gene shows a high degree of sequence homology with the mouse and human Sox4 genes, particularly in the HMG-like DNA binding domain and at the carboxy terminus. Furthermore, our in situ hybridization studies document an expression pattern during embryonic development that is very similar to that described for the mouse, particularly with regards to expression in the developing heart. However, abundant expression was also detected in tissues of neural crest origin including pharyngeal arch and craniofacial mesoderm, supporting a potential primary role in neural crest cardiac pathology previously detected in Sox4 mutant mice. Furthermore, a reciprocal pattern of Sox4 and Sox11 expression in the developing neural tube was detected in the chicken compared to that seen in the mouse. These studies suggest that Sox4 plays an important and conserved role in the embryonic development of these structures in the chicken as well as in the mouse and lay the foundation for future studies of the role of Sox4 during critical events of organogenesis.

Bone morphogenetic protein 2 induces placental growth factor in mesenchymal stem cells

Bone-forming osteoblasts differentiate from pluripotent mesenchymal stem cells (MSCs) in a multistage process that can be modeled in vitro using MSCs isolated from adult human trabecular bone or bone marrow. To identify new genes involved in osteoblast differentiation, we have performed large-scale gene expression profiling using high-density cDNA microarrays in primary human MSCs treated with the known osteogenic agent bone morphogenetic protein 2 (BMP-2). The vascular endothelial growth factor (VEGF) family member placental growth factor (PlGF) was found as an early regulated gene whose induction was already detected after 2 h treatment with BMP-2. Tissue distribution analysis of PlGF mRNA expression using microarrays revealed a very restricted expression of PlGF only in BMP-2-treated MSCs and in placenta as expected. Ribonuclease protection assay (RPA) confirmed the induction of PlGF and showed preferential expression of the PlGF-1 isoform over PLGF-2 in MSCs and MG63 cells. BMP-2 stimulated PlGF expression in MG63 cells with an EC50 of about 50 ng/ml and mRNA levels peaked between 24 and 32 h after stimulation. Furthermore, induction of PlGF by BMP-2 appeared specific, as other osteogenic agents including vitamin D3, transforming growth factor beta, and basic fibroblast growth factor were inactive. BMP-2 stimulated PlGF secretion from MG63 and MSC cells, but PlGF had no effect on MSC proliferation and osteoblastic differentiation. Based on the known function of PlGF in the recruitment of endothelial and hematopoietic stem cells, these results suggest a paracrine role for MSC-derived PlGF in the angiogenesis and hematopoiesis that accompany BMP-2-induced bone formation.

A gene expression profile for endochondral bone formation: oligonucleotide microarrays establish novel connections between known genes and BMP-2-induced bone formation in mouse quadriceps

Endochondral bone formation has been fairly well characterized from a morphological perspective and yet this process remains largely undefined at molecular and biochemical levels. In vitro and in vivo studies have shown that human bone morphogenetic protein-2 (hBMP-2) is an important developmental growth and differentiation factor, capable of inducing ectopic bone formation in vivo. This study evaluated several aspects of the osteogenic effect of hBMP-2 protein injected into quadriceps of female C57B1/6J SCID mice. Mice were euthanized 1, 2, 3, 4, 7, and 14 days postinjection and muscles were collected for several methods of analysis. Hematoxylin and eosin-stained sections of muscles injected with formulation buffer showed no evidence of osteogenesis. In contrast, sections of muscles injected with hBMP-2 showed evidence of endochondral bone formation that progressed to mineralized bone by day 14. In addition, radiographs of mice injected with hBMP-2 showed that much of the quadriceps muscle had undergone mineralization by day 14. Labeled mRNA solutions were prepared and hybridized to oligonucleotide arrays designed to monitor approximately 1300 murine, full-length genes. Changes in gene expression associated with hBMP-2 were determined from time-matched comparisons between buffer and hBMP-2 samples. A gene expression profile was created for 215 genes that showed greater than 4-fold changes at one or more of the indicated time points. One hundred twenty-two of these genes have previously been associated with bone or cartilage metabolism and showed significant increases in expression, e.g., aggrecan (Agc1), runt related transcription factor 2 (Runx2), bone Gla protein 1 (Bglap1), and procollagens type II (Col2a1) and X (Col10a1). In addition, there were 93 genes that have not been explicitly associated with bone or cartilage metabolism. Two of these genes, cytokine receptor-like factor-1 (Crlf1) and matrix metalloproteinase 23 (Mmp23), showed peak changes in gene expression of 15- and 40-fold on days 4 and 7, respectively. In situ hybridizations of muscle sections showed that Mmp23 and Crlf1 mRNAs were expressed in chondrocytes and osteoblasts, suggesting a role for both proteins in some aspect of cartilage or bone formation. In conclusion, oligonucleotide arrays enabled a broader view of endochondral bone formation than has been reported to date. An increased understanding of the roles played by these gene products will improve our understanding of skeletogenesis, fracture repair, and pathological conditions such as osteoporosis.

Gene array analysis of bone morphogenetic protein type I receptor-induced osteoblast differentiation

The genomic response to BMP was investigated by ectopic expression of activated BMP type I receptors in C2C12 myoblast using cDNA microarrays. Novel BMP receptor target genes with possible roles in inhibition of myoblast differentiation and stimulation of osteoblast differentiation were identified.

INTRODUCTION

Bone morphogenetic proteins (BMPs) have an important role in controlling mesenchymal cell fate and mediate these effects by regulating gene expression. BMPs signal through three distinct specific BMP type I receptors (also termed activin receptor-like kinases) and their downstream nuclear effectors, termed Smads. The critical target genes by which activated BMP receptors mediate change cell fate are poorly characterized.

MATERIALS AND METHODS

We performed transcriptional profiling of C2C12 myoblasts differentiation into osteoblast-like cells by ectopic expression of three distinct constitutively active (ca)BMP type I receptors using adenoviral gene transfer. Cells were harvested 48 h after infection, which allowed detection of both early and late response genes. Expression analysis was performed using the mouse GEM1 microarray, which is comprised of approximately 8700 unique sequences. Hybridizations were performed in duplicate with a reverse fluor labeling. Genes were considered to be significantly regulated if the p value for differential expression was less than 0.01 and inverted expression ratios per duplicate successful reciprocal hybridizations differed by less than 25%.

RESULTS AND CONCLUSIONS

Each of the three caBMP type I receptors stimulated equal levels of R-Smad phosphorylation and alkaline phosphatase activity, an early marker for osteoblast differentiation. Interestingly, all three type I receptors induced identical transcriptional profiles; 97 genes were significantly upregulated and 103 genes were downregulated. Many extracellular matrix genes were upregulated, muscle-related genes downregulated, and transcription factors/signaling components modulated. In addition to 41 expressed sequence tags without known function and a number of known BMP target genes, including PPAR-gamma and fibromodulin, a large number of novel BMP target genes with an annotated function were identified, including transcription factors HesR1, ITF-2, and ICSBP, apoptosis mediators DRP-1 death kinase and ZIP kinase, IkappaB alpha, Edg-2, ZO-1, and E3 ligase Dactylin. These target genes, some of them unexpected, offer new insights into how BMPs elicit biological effects, in particular into the mechanism of inhibition of myoblast differentiation and stimulation of osteoblast differentiation.

Phenotype discovery by gene expression profiling: mapping of biological processes linked to BMP-2-mediated osteoblast differentiation

Understanding physiological control of osteoblast differentiation necessitates characterization of the regulatory signals that initiate the events directing a cell to lineage commitment and establishing competency for bone formation. The bone morphogenetic protein, BMP-2, a member of the TGFbeta superfamily, induces osteoblast differentiation and functions through the Smad signal transduction pathway during in vivo bone formation. However, the molecular targets of BMP-mediated gene transcription during the process of osteoblast differentiation have not been comprehensively identified. In the present study, BMP-2 responsive factors involved in the early stages of commitment and differentiation to the osteoblast phenotype were analyzed by microarray gene expression profiling in samples ranging from 1 to 24 h following BMP-2 dependent differentiation of C2C12 premyoblasts into the osteogenic lineage. A total of 1,800 genes were responsive to BMP-2 and expression was modulated from 3- to 14-fold for less than 100 genes during the time course. Approximately 50% of these 100 genes are either up- or downregulated. Major events associated with phenotypic changes towards the osteogenic lineage were identified from hierarchical and functional clustering analyses. BMP-2 immediately responsive genes (1-4 h), which exhibited either transient or sustained expression, reflect activation and repression of non-osseous BMP-2 developmental systems. This initial response was followed by waves of expression of nuclear proteins and developmental regulatory factors including inhibitors of DNA binding, Runx2, C/EBP, Zn finger binding proteins, forkhead, and numerous homeobox proteins (e.g., CDP/cut, paired, distaless, Hox) which are expressed at characterized stages during osteoblast differentiation. A sequential profile of genes mediating changes in cell morphology, cell growth, and basement membrane formation is observed as a secondary transient early response (2-8 h). Commitment to the osteogenic phenotype is recognized by 8 h, reflected by downregulation of most myogenic-related genes and induction of a spectrum of signaling proteins and enzymes facilitating synthesis and assembly of an extracellular skeletal environment. These genes included collagens Type I and VI and the small leucine rich repeat family of proteoglycans (e.g., decorin, biglycan, osteomodulin, fibromodulin, and osteoadherin/osteoglycin) that reached peak expression at 24 h. With extracellular matrix development, the bone phenotype was further established from 16 to 24 h by induction of genes for cell adhesion and communication and enzymes that organize the bone ECM. Our microarray analysis resulted in the discovery of a class of genes, initially described in relation to differentiation of astrocytes and oligodendrocytes that are functionally coupled to signals for cellular extensions. They include nexin, neuropilin, latexin, neuroglian, neuron specific gene 1, and Ulip; suggesting novel roles for these genes in the bone microenvironment. This global analysis identified a multistage molecular and cellular cascade that supports BMP-2-mediated osteoblast differentiation.

Transcriptional profiling of human osteoblast differentiation

Osteoblast differentiation is a key aspect of bone formation and remodeling. To further our understanding of the differentiation process, we have developed a collection of conditionally immortalized adult human osteoblast cell lines representing discrete stages of differentiation. To evaluate changes in gene expression associated with differentiation, polyA((+)) RNA from pre-osteoblasts, early and late osteoblasts, and pre-osteocytes was subjected to gene chip analysis using the Affymetrix Hu6800 chip in conjunction with an Affymetrix custom chip enriched in bone and cartilage cDNAs. Overall, the expression of 47 genes was found to change threefold or more on both chips between the pre-osteoblastic and pre-osteocytic stages of differentiation. Many of the observed differences, including down-regulation of collagen type I and collagen-processing enzymes, reflect expected patterns and support the relevance of our results. Other changes have not been reported and offer new insight into the osteoblast differentiation process. Thus, we observed regulation of factors controlling cell cycle and proliferation, reflecting decreased proliferation, and increased apoptosis in pre-osteocytic cells. Elements maintaining the cytoskeleton, extracellular matrix, and cell-cell adhesion also changed with differentiation reflecting profound alterations in cell architecture associated with the differentiation process. We also saw dramatic down-regulation of several components of complement and other immune response factors that may be involved in recruitment and differentiation of osteoclasts. The decrease in this group of genes may provide a mechanism for controlling bone remodeling of newly formed bone. Our screen also identified several signaling proteins that may control osteoblast differentiation. These include an orphan nuclear receptor DAX1 and a small ras-related GTPase associated with diabetes, both of which increased with increasing differentiation, as well as a high mobility group-box transcription factor, SOX4, that was down-regulated during differentiation. In summary, our study provides a comprehensive transcriptional profile of human osteoblast differentiation and identifies several genes of potential importance in controlling differentiation of osteoblasts.

Role of RANK ligand in mediating increased bone resorption in early postmenopausal women

Studies in rodents have implicated various cytokines as paracrine mediators of increased osteoclastogenesis during estrogen deficiency, but increases in RANKL, the final effector of osteoclastogenesis, have not been demonstrated. Thus, we isolated bone marrow mononuclear cells expressing RANKL on their surfaces by two-color flow cytometry using FITC-conjugated osteoprotegerin-Fc (OPG-Fc-FITC) as a probe. The cells were characterized as preosteoblastic marrow stromal cells (MSCs), T lymphocytes, or B lymphocytes by using Ab's against bone alkaline phosphatase (BAP), CD3, and CD20, respectively, in 12 premenopausal women (Group A), 12 early postmenopausal women (Group B), and 12 age-matched, estrogen-treated postmenopausal women (Group C). Fluorescence intensity of OPG-Fc-FITC, an index of the surface concentration of RANKL per cell, was increased in Group B over Groups A and C by two- to threefold for MSCs, T cells, B cells, and total RANKL-expressing cells. Moreover, in the merged groups, RANKL expression per cell correlated directly with the bone resorption markers, serum C-terminal telopeptide of type I collagen and urine N-telopeptide of type I collagen, in all three cell types and inversely with serum 17beta-estradiol for total RANKL-expressing cells. The data suggest that upregulation of RANKL on bone marrow cells is an important determinant of increased bone resorption induced by estrogen deficiency.

Temporal gene expression changes during adipogenesis in human mesenchymal stem cells

Human bone marrow mesenchymal stem cells (hMSCs) give rise to adipocytes in response to adipogenic hormones. An in-house cDNA microarray representing 3400 genes was employed to characterize the modulation of genes involved in this process. A total of 197 genes showed temporal gene expression changes during adipogenesis, including genes encoding transcriptional regulators and signaling molecules. Semi-quantitative RT-PCR analyses confirmed differential expression at the transcriptional level of several genes identified by cDNA microarray screening. Cluster analysis of the genes regulated during the late phase (from day 7 to day 14) of hMSC adipogenesis indicated that these changes are well correlated with data previously reported for murine preadipocytes. However, during the early phase (day 1-day 5), the modulations of genes differed from those reported for the preadipocytes. These data provide novel information on the molecular mechanisms required for lineage commitment and maturation accompanying adipogenesis of hMSC.

Genomic approaches to identifying transcriptional regulators of osteoblast differentiation

Recent microarray studies of mouse and human osteoblast differentiation in vitro have identified novel transcription factors that may be important in the establishment and maintenance of differentiation. These findings help unravel the pattern of gene-expression changes that underly the complex process of bone formation.

Regulation of BMP-induced transcription in cultured human bone marrow stromal cells

BACKGROUND

Adherent bone marrow stromal cells are inducible osteoprogenitors, giving rise to cells expressing osteoblast markers including alkaline phosphatase, osteopontin, osteocalcin, and bone sialoprotein. However, the potency of inducers varies in a species-specific manner. Glucocorticoids such as dexamethasone induce alkaline phosphatase activity in both human and rat mesenchymal stem cells, while mouse bone marrow stromal cells are refractory to dexamethasone-induced alkaline phosphatase activity. In contrast, BMP induces alkaline phosphatase activity in both mouse and rat bone marrow stromal cells, while BMP effects on human bone marrow stromal cells are poorly characterized.

METHODS

Bone marrow samples were isolated from patients undergoing hip replacement. Mononuclear marrow cells were cultured and grown to confluence without or with 10 (-7) M dexamethasone. Cells from each isolate were passaged into medium containing 100 micro g/mL ascorbate phosphate and treated with dexamethasone, 100 ng/mL BMP, or no inducer. At day 6, alkaline phosphatase activity was assayed, and RNA was prepared for mRNA analyses by real-time polymerase chain reaction.

RESULTS

Bone marrow stromal cells from twenty-four of twenty-six patients showed no significant osteogenic response to BMP-2, 4, or 7 as determined by alkaline phosphatase induction. However, BMPs induced elevated levels of other genes associated with osteogenesis such as bone sialoprotein and osteopontin as well as BMP-2 and noggin. If primary cultures of human bone marrow stromal cells were pretreated with dexamethasone, BMP-2 treatment of first-passage cells induced alkaline phosphatase in approximately half of the isolates, and significantly greater induction was seen in cells from males. Dexamethasone treatment, like BMP treatment, also increased expression of the BMP-binding protein noggin.

CONCLUSIONS

Most human femur bone marrow stromal cell samples appear incapable of expressing elevated alkaline phosphatase levels in response to BMPs. Since BMP treatment induced expression of several other BMP-regulated genes, the defect in alkaline phosphatase induction is presumably not due to impaired BMP signaling. We hypothesize that the mechanism by which BMPs modulate alkaline phosphatase expression is indirect, involving a BMP-regulated transcription factor for alkaline phosphatase expression that is controlled differently in humans and rodents.

Integration of the non-genomic and genomic actions of estrogen. Membrane-initiated signaling by steroid to transcription and cell biology

Estrogen binds to receptors that translocate to the plasma membrane and to the nucleus. The rapid, non-genomic actions of this sex steroid are attributed to membrane action, while gene transcription occurs through nuclear receptor function. However, gene transcription can also result from estrogen signaling initiated at the membrane, but the relative importance of this mechanism is not known. In vascular endothelial cells (EC), estradiol (E(2)) activates several kinase cascades, including phosphatidylinositol 3-phosphate (PI3K)/Akt, a signaling pathway that impacts EC biology. We determined here by DNA microarray that 40-min exposure to E(2) significantly increased 250 genes in EC, up-regulation that was substantially prevented by the PI3K inhibitor, LY294002. This coincided with maximum E(2)-induced PI3K activity at 15-30 min. An important vascular gene strongly up-regulated by E(2) in our array produces cyclooxygenase-2 (Cox-2). In cultured EC, E(2) induced both Cox-2 gene expression and new Cox-2 protein synthesis by 40 and 60 min, respectively, and rapidly stimulated the secretion of prostaglandins PGI(2) and PGE(2). The up-regulation of gene expression reflected transcriptional transactivation, shown using Cox-2 promoter/luciferase reporters in the EC. Soluble inhibitors or dominant negative constructs for PI3K and Akt prevented all these actions of E(2). Functionally, EC migration was induced by the sex steroid, and this was significantly reversed by NS-398, a Cox-2 inhibitor. Gene transcription and cell biological effects of estrogen emanate from rapid and specific signaling, integrating cell surface and nuclear actions of this steroid.

Comprehensive microarray analysis of bone morphogenetic protein 2-induced osteoblast differentiation resulting in the identification of novel markers for bone development

Osteoblasts are cells responsible for matrix deposition during bone development and although temporal expression of many genes has been related to osteoblast differentiation, a complete description of osteoblast-specific gene regulation will lead to a better understanding of osteoblast function. In this study, microarray technology was used to analyze gene expression on a broad scale during osteoblast differentiation. Expression analysis of 9596 sequences revealed 342 genes and expressed sequence tags (ESTs) to be modulated differentially during a time course experiment in which murine C2C12 mesenchymal progenitor cells were induced to differentiate into mature osteoblasts by treatment with bone morphogenetic protein 2 (BMP-2). By means of hierarchical clustering, these genes were grouped by similarities in their expression profiles, resulting in subsets of early, intermediate, and late response genes, which are representative of the distinct stages of osteoblast differentiation. To identify new bone markers, the bone specificity of the late response genes was determined by comparing BMP-induced expression in C2C12 and MC3T3 osteoblasts with that in NIH3T3 fibroblasts. This resulted in the identification of nine novel genes and ESTs that were induced specifically in osteoblasts, in addition to the well-known markers ALP and osteocalcin. For at least one of these novel genes, Wnt inhibitory factor 1, and two of the ESTs, expression in developing bone was verified in vivo by in situ hybridization of E16.5 mouse embryos. In conclusion, by a combination of in vitro and in vivo screening approaches, a set of new genes related to osteoblast differentiation and skeletal development has been identified.

Cell-type-dependent up-regulation of in vitro mineralization after overexpression of the osteoblast-specific transcription factor Runx2/Cbfal

Functional expression of the transcriptional activator Runx2/Cbfal is essential for osteoblastic differentiation and bone formation and maintenance. Forced expression of Runx2 in nonosteoblastic cells induces expression of osteoblast-specific genes, but the effects of Runx2 overexpression on in vitro matrix mineralization have not been determined. To examine whether exogenous Runx2 expression is sufficient to direct in vitro mineralization, we investigated sustained expression of Runx2 in nonosteoblastic and osteoblast-like cell lines using retroviral gene delivery. As expected, forced expression of Runx2 induced several osteoblast-specific genes in NIH3T3 and C3H10T1/2 fibroblasts and up-regulated expression in MC3T3-E1 immature osteoblast-like cells. However, Runx2 expression enhanced matrix mineralization in a cell-type-dependent manner. NIH3T3 and IMR-90 fibroblasts overexpressing Runx2 did not produce a mineralized matrix, indicating that forced expression of Runx2 in these nonosteogenic cell lines is not sufficient to direct in vitro mineralization. Consistent with the pluripotent nature of the cell line, a fraction (25%) of Runx2-expressing C3H10T1/2 fibroblast cultures produced mineralized nodules in a viral supernatant-dependent manner. Notably, bone sialoprotein (BSP) gene expression was detected at significantly higher levels in mineralizing Runx2-infected C3H10T1/2 cells compared with Runx2-expressing cultures which did not mineralize. Treatment of Runx2-infected C3H10T1/2 cultures with dexamethasone enhanced osteoblastic phenotype expression, inducing low levels of mineralization independent of viral supernatant. Finally, Runx2 overexpression in immature osteoblast-like MC3T3-E1 cells resulted in acceleration and robust up-regulation of matrix mineralization compared with controls. These results suggest that, although functional Runx2 is essential to multiple osteoblast-specific activities, in vitro matrix mineralization requires additional tissue-specific cofactors, which supplement Runx2 activity.

Behavior of osteoblast, adipocyte, and myoblast markers in genome-wide expression analysis of mouse calvaria primary osteoblasts in vitro

Several genes, such as alkaline phosphatase, osteocalcin, and Cbfa1/Osf2, are known to be regulated during osteoblastic differentiation and are commonly used as "osteoblast markers" for in vitro or in vivo studies. The number of these genes is very limited, however, and it is of major interest to identify new genes that are activated or repressed during the process of osteoblast differentiation and bone formation as well as to extend the available information on gene families relevant to this particular differentiation pathway. To identify such genes, we have implemented a genome-wide analysis by determining changes in expression levels of 27,000 genes during in vitro differentiation of primary osteoblasts isolated from mouse calvaria. This study focuses on the description of the analytical and filtering process applied; on the transcriptional analysis of well-established "bone," "adipocyte," and "muscle" pathway markers; and on a description of the regulation profiles for genes recently described in the Skeletal Gene Database. We also demonstrate that new array technologies constitute reliable and powerful tools to monitor the transcription of genes involved in osteoblastic differentiation, allowing a more integrated vision of the biological pathways regulated during osteoblast commitment, differentiation, and function.

Signal transductions induced by bone morphogenetic protein-2 and transforming growth factor-beta in normal human osteoblastic cells

Transforming growth factor beta (TGF-beta) activates Ras/MAPK signaling in many cell types. Because TGF-beta and BMP-2 exert similar effects, we examined if this signaling is stimulated by both factors and analyzed the relationship between this signaling and the Smads in osteoblasts. BMP-2 and TGF-beta stimulated Ras, MAPK, and AP-1 activities. The DNA binding activities of c-Fos, FosB/Delta FosB, Fra-1, Fra-2, and JunB were up-regulated whereas JunD activity was decreased. c-Fos, FosB/Delta FosB, and JunB were associated with Smad4. The stimulation of AP-1 by BMP-2 and TGF-beta was dependent on Smad signaling, and anti-Smad4 antibody interfered with AP-1 activity. Thus, BMP-2 and TGF-beta activate both Ras/MAPK/AP-1 and Smad signaling in osteoblasts with Smads modulating AP-1 activity. To determine the roles of MAPK in BMP-2 and TGF-beta function, we analyzed the effect of ERK and p38 inhibitors on the regulation of bone matrix protein expression and JunB and JunD levels by these two factors. ERK and p38 mediated TGF-beta suppression of osteocalcin and JunD as well as stimulation of JunB. p38 was essential in BMP-2 up-regulation of type I collagen, fibronectin, osteopontin, osteocalcin, and alkaline phosphatase activity whereas ERK mediated BMP-2 stimulation of fibronectin and osteopontin. Thus, ERK and p38 differentially mediate TGF-beta and BMP-2 function in osteoblasts.