Identification of a novel TGF-beta-regulated gene encoding a putative zinc finger protein in human osteoblasts

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HER-2 overexpression differentially alters transforming growth factor-beta responses in luminal versus mesenchymal human breast cancer cells

Introduction

Amplification of the HER-2 receptor tyrosine kinase has been implicated in the pathogenesis and aggressive behavior of approximately 25% of invasive human breast cancers. Clinical and experimental evidence suggest that aberrant HER-2 signaling contributes to tumor initiation and disease progression. Transforming growth factor beta (TGF-β) is the dominant factor opposing growth stimulatory factors and early oncogene activation in many tissues, including the mammary gland. Thus, to better understand the mechanisms by which HER-2 overexpression promotes the early stages of breast cancer, we directly assayed the cellular and molecular effects of TGF-β1 on breast cancer cells in the presence or absence of overexpressed HER-2.

Methods

Cell proliferation assays were used to determine the effect of TGF-β on the growth of breast cancer cells with normal or high level expression of HER-2. Affymetrix microarrays combined with Northern and western blot analysis were used to monitor the transcriptional responses to exogenous TGF-β1 in luminal and mesenchymal-like breast cancer cells. The activity of the core TGF-β signaling pathway was assessed using TGF-β1 binding assays, phospho-specific Smad antibodies, immunofluorescent staining of Smad and Smad DNA binding assays.

Results

We demonstrate that cells engineered to over-express HER-2 are resistant to the anti-proliferative effect of TGF-β1. HER-2 overexpression profoundly diminishes the transcriptional responses induced by TGF-β in the luminal MCF-7 breast cancer cell line and prevents target gene induction by a novel mechanism that does not involve the abrogation of Smad nuclear accumulation, DNA binding or changes in c-myc repression. Conversely, HER-2 overexpression in the context of the mesenchymal MDA-MB-231 breast cell line potentiated the TGF-β induced pro-invasive and pro-metastatic gene signature.

Conclusion

HER-2 overexpression promotes the growth and malignancy of mammary epithelial cells, in part, by conferring resistance to the growth inhibitory effects of TGF-β. In contrast, HER-2 and TGF-β signaling pathways can cooperate to promote especially aggressive disease behavior in the context of a highly invasive breast tumor model.

Regulation of osteoblastic phenotype and gene expression by hop-derived phytoestrogens

Certain plant-derived compounds show selective estrogen receptor modulator (SERM) activity and may therefore be an alternative to the conventional hormone replacement therapy, which prevents osteoporosis but is also associated with an increased risk of breast and endometrial cancers. In the current study, we tested the effects of the hop-derived compounds 8-prenylnaringenin, 6-prenylnaringenin, xanthohumol and isoxanthohumol (1) to modulate markers of differentiation and gene expression in osteoblasts and (2) to regulate proliferation in MCF-7 breast cancer cells. Additionally, we analyzed the ER-binding affinities of these hop compounds as well as the ER-mediation of their effects. Bone-forming activity and ER-subtype specificity were investigated by measuring alkaline phosphatase (AP) activity in hFOB/ERalpha cells and regulation of gene transcription for AP, interleukin-6, pS2 and von Willebrand factor (VWF) in U-2 OS/ERalpha and U-2 OS/ERbeta cells. Our results demonstrate that AP, pS2 and VWF mRNA levels are significantly increased by the compounds in an estrogen-like manner via both ERalpha and ERbeta, while IL-6 is down-regulated in U-2 OS/ERalpha cells. Consistently, AP enzymatic activity is up-regulated by all compounds in hFOB/ERalpha9 cells. Depending on their concentration, all compounds show proliferative effects in MCF-7 cells. Except for 8-PN the hop constituents display an ERbeta-preference. Reversal of estrogen-specific AP-induction in Ishikawa cells indicates an ER-regulated mechanism. Finally, the flavonoids display cytotoxic effects only at high concentrations (> or =10(-4)M). In summary, we have demonstrated for the first time that specific phytoestrogen compounds found in hop extracts exert estrogen-like activities on bone metabolism. Regarding a potential for use in osteoporosis-prevention therapy, the dosage of a phytoestrogen, which is taken, will play an important role concerning a desired in vivo profile.

Functional Characterization of JMJD2A, a Histone Deacetylase- and Retinoblastoma-binding Protein

To effectively direct targeted repression, the class I histone deacetylases (HDACs) associate with many important regulatory proteins. In this paper we describe the molecular characterization of a member of the Jumonji domain 2 (JMJD2) family of proteins, and demonstrate its binding to both class I HDACs and the retinoblastoma protein (pRb). JMJD2 proteins are characterized by the presence of two leukemia-associated protein/plant homeodomain (LAP/PHD) zinc fingers, one JmjN, one JmjC (containing an internal retinoblastoma-binding protein 2 (RBBP2)-like sequence), and two Tudor domains. The first member of this group, JMJD2A, is widely expressed in human tissues and cell lines, and high endogenous expression of JMJD2A mRNA was found in several cell types, including human T-cell lymphotropic virus 1 (HTLV-1)-infected cell lines. JMJD2A and JMJD2B exhibit cell type-specific responses to the HDAC inhibitor trichostatin A. We show that the JMJD2A protein associates in vivo with pRb and class I HDACs, and mediates repression of E2F-regulated promoters. In HTLV-1 virus-infected cells, we find that JMJD2A binds to the viral Tax protein. Antibodies to JMJD2A recognize the native protein but also a half-sized protein fragment, the latter up-regulated in THP-1 cells during the G(2)/M phase of the cell cycle. The ability of JMJD2A to associate with pRb and HDACs and potentiate pRb-mediated repression of E2F-regulated promoters implies an important role for this protein in cell proliferation and oncogenesis.

Balance of pro-apoptotic transforming growth factor-β and anti-apoptotic insulin effects in the control of cell death in the postnatal mouse retina

Transforming growth factor (TGF)-beta and insulin display opposite effects in regulating programmed cell death during vertebrate retina development; the former induces apoptosis while the latter prevents it. In the present study we investigated coordinated actions of TGF-beta and insulin in an organotypic culture system of early postnatal mouse retina. Addition of exogenous TGF-beta resulted in a significant increase in cell death whereas exogenous insulin attenuated apoptosis and was capable of blocking TGF-beta-induced apoptosis. This effect appeared to be modulated via insulin-induced transcriptional down-regulation of TGF-beta receptor II levels. The analysis of downstream signalling molecules also revealed opposite effects of both factors; insulin provided survival signalling by increasing the level of anti-apoptotic Bcl-2 protein expression and phosphorylation and down-regulating caspase 3 activity whereas pro-apoptotic TGF-beta signalling reduced Bcl-2 mRNA levels and Bcl-2 phosphorylation and induced the expression of TGF-induced immediate-early gene (TIEG), a Krüppel-like zinc-finger transcription factor, mimicking TGF-beta activity.

Identification, Characterization, and Functional Analysis of Sp1 Transcript Variants Expressed in Germ Cells During Mouse Spermatogenesis1

The SP family of zinc-finger transcription factors are important mediators of selective gene activation during embryonic development and cellular differentiation. SP-binding GC-box domains are common cis-regulatory elements present in the promoters of several genes expressed in a developmentally specific manner in differentiating mouse germ cells. Four Sp1 cDNAs were isolated from a mouse pachytene spermatocyte cDNA library and characterized by DNA sequence analysis. Northern blot studies revealed that these cDNAs corresponded to 3 full-length Sp1 transcripts (4.1, 3.7, and 3.2 kilobases [kb]) and an additional 1.4-kb 5'-truncated Sp1 transcript that are temporally expressed during spermatogenesis. Quantitative real-time polymerase chain reaction studies verified that the highest levels of Sp1 transcript expression of 4.1, 3.7, and 3.2 kb occur in the primary spermatocytes. The spatial and temporal expression patterns of these Sp1 transcripts and their encoded 60-kDa and 90-kDa SP1 proteins were demonstrated using in situ hybridization and immunohistochemical analyses. To assess the transcriptional properties of these SP1 transcription factors, SP-deficient Drosophila SL2 cells were stably transfected with the respective Sp1 cDNA expression vectors and cotransfected with either Ldh2, Ldh3, or Creb promoter/luciferase reporter constructs. The levels of SP-mediated luciferase expression observed depended on the structure of the glutamine-rich transactivation domains and the number of GC-box elements present in the respective promoters. The alterations observed in germ cells in the patterns of expression of the Sp1 transcripts encoding the 60-kDa and 90-kDa SP1 isoforms suggest that these SP1 factors may be involved in mediating stage-specific and cell type-specific gene expression during mouse spermatogenesis.

TIEG1 null mouse-derived osteoblasts are defective in mineralization and in support of osteoclast differentiation in vitro

Transforming growth factor beta-inducible early gene 1 (TIEG1) is a member of the Kruppel-like transcription factor family. To understand the physiological role of TIEG1, we generated TIEG(-/-) (null) mice and found that the TIEG(-/-) mice had increased osteoblast numbers with no increased bone formation parameters. However, when calvarial osteoblasts (OBs) were isolated from neonatal TIEG(-/-) and TIEG(+/+) mice and cultured in vitro, the TIEG(-/-) cells displayed reduced expression of important OB differentiation markers. When the OBs were differentiated in vitro by treatment with bone morphogenic protein 2, the OBs from TIEG(+/+) calvaria displayed several mineralized nodules in culture, whereas those from TIEG(-/-) mice showed no nodules. To characterize the OBs' ability to support osteoclast differentiation, the OBs from TIEG(+/+) and TIEG(-/-) mice were cultured with marrow and spleen cells from TIEG(+/+) mice. Significantly fewer osteoclasts developed when TIEG(-/-) OBs were used to support osteoclast differentiation than when TIEG(+/+) OBs were used. Examination of gene expression in the TIEG(-/-) OBs revealed decreased RANKL and increased OPG expression compared to TIEG(+/+) OBs. The addition of RANKL to these cocultures only partially restored the ability of TIEG(-/-) OBs to support osteoclast differentiation, whereas M-CSF alone or combined with RANKL had no additional effect on osteoclast differentiation. We conclude from these data that TIEG1 expression in OBs is critical for both osteoblast-mediated mineralization and osteoblast support of osteoclast differentiation.

The Leukocyte Integrin Gene CD11d Is Repressed by Gut-enriched Kruppel-like Factor 4 in Myeloid Cells

The myeloid-specific leukocyte integrin CD11d encodes the alphaD subunit for the alphaDbeta2 receptor. A yeast one-hybrid screen showed that a longer isoform of gut-enriched Kruppel-like factor 4 (GKLF) we term GKLFa interacts with the CD11d promoter. Purified GST-GKLFa protein was shown to bind within the -61 to -44 region that overlaps a binding site for the CD11d transcriptional activators Sp1 and transforming growth factor beta-inducible early gene-1 (TIEG1). Transfection of GKLF/GKLFa in myeloid cells reduced CD11d promoter activity, whereas, down-regulation of GKLF/GKLFa with small interfering RNAs led to up-regulation of CD11d expression. Differentiation of myeloid cells with phorbol ester led to activation of the CD11d promoter and reduced occupancy of the promoter by GKLF/GKLFa but an increased occupancy by TIEG1 in vivo. Binding of GKLF/GKLFa, Sp1, and TIEG1 to the CD11d promoter in vivo is dependent on their zinc finger DNA binding domains. GKLFa physically associates with the histone deacetylases (HDAC) 1 and 2, and both HDACs are bound to the CD11d promoter in vivo but released after exposure of myeloid cells to phorbol ester suggesting that GKLF/GKLFa recruits HDACs to effect repression.

Differential gene expression of TGF beta inducible early gene (TIEG), Smad7, Smad2 and Bard1 in normal and malignant breast tissue

TGF beta/Smad signaling pathway members are potent tumor suppressors for many types of cancers. We hypothesize that breast tumors differentially express these genes and that this expression pattern plays a role in the proliferation of breast cancer. We examined the mRNA levels of TIEG, Smad7, Smad2, and Bard1 using real-time RT/PCR in 14 normal breast, five non-invasive, 57 invasive (including 29 with outcome data), and five metastatic breast tumor tissues. TIEG and Smad7 mRNA levels were lower in non-invasive tumors compared to normal breast tissues. TIEG, Bard1, and Smad2 mRNA levels were lower in invasive cancers compared to normal breast tissues. In addition, TIEG, Smad2, and Bard1, provided discriminatory ability to potentially distinguish between normal and tumor samples, N- and N+ tumors, and N-/good (no recurrence for at least 5 years) and N-/bad (recurrence within 3 years) outcome patients. TIEG mRNA levels accurately discriminated between normal breast tissue and primary tumors with a sensitivity and specificity of 96 and 93%, respectively. TIEG, in combination with Smad2, distinguished between N+ and N- primary tumors with a sensitivity and specificity of 75 and 85%, respectively. TIEG in combination with Bard1 discriminated between N-/bad outcome from N-/good tumors with a sensitivity and specificity of 83 and 82%, respectively. Our results support the hypothesis that the differential gene expression of TIEG, Smad2, and Bard1, which are tumor suppressor genes, plays a significant role in the proliferation of breast cancer. Further investigation is necessary to validate the ability of these genes to discriminate between different populations of breast cancer patients.

Genomic structure and cloning of two transcript isoforms of human Sp8

Background

The Specificity proteins (Sp) are a family of transcription factors that have three highly conserved zinc-fingers located towards the carboxy-terminal that bind GC-boxes and assist in the initiation of gene transcription. Human Sp1-7 genes have been characterized. Recently, the phenotype of Sp8 null mice has been described, being tailless and having severe truncation of both fore and hind limbs. They also have malformed brains with defective closure of the anterior and posterior neuropore during brain development.

Results

The human Sp8 gene is a three-exon gene that maps to 7p21.3, close to the related Sp4 gene. From an osteosarcoma cell line we cloned two transcript variants that use two different first exons and have a common second exon. One clone encodes a 508-residue protein, Sp8L (isoform 1) and the other a shorter 490-residue protein, Sp8S (isoform 2). These two isoforms are conserved being found also in mice and zebrafish. Analysis of the Sp8L protein sequence reveals an amino-terminal hydrophobic Sp-motif that is disrupted in Sp8S, a buttonhead box and three C₂H₂ zinc-fingers. Sp8 mRNA expression was detected in a wide range of tissues at a low level, with the highest levels being found in brain. Treatment of the murine pluripotent cell line C3H10T1/2 with 100 ng/mL BMP-2 induced Sp8 mRNA after 24 hours.

Conclusions

There is conservation of the two Sp8 protein isoforms between primates, rodents and fish, suggesting that the isoforms have differing roles in gene regulation. Sp8 may play a role in chondrogenic/osteoblastic differentiation in addition to its role in brain and limb development.

Zinc finger transcription factors in skeletal development

Cellular and molecular processes that regulate the development of skeletal tissues resemble those required for regeneration. Given the prevalence of degenerative skeletal disorders in an increasingly aging population, the molecular mechanisms of skeletal development must be understood in detail if novel strategies are to be developed in regenerative medicine. Research in this area over the past decade has revealed that cell differentiation is largely controlled at the level of gene transcription, which in turn is regulated by transcription factors. Transcription factors usually recognize and bind to specific DNA sequences in the promoter of target genes via characteristic DNA-binding domains. Although the gene family containing C2H2 zinc fingers as DNA-binding motifs is the largest family of transciptional regulators, with several hundred individual members in mammals, only a small but increasing number of zinc finger genes have been implicated in bone, cartilage, or tooth development. These zinc finger proteins (ZFPs) contain multiple structural motifs that require zinc to maintain their structural integrity and function. Interestingly, zinc deficiency is known to result in skeletal growth retardation and has been identified as a risk factor in the pathogenesis of osteoporosis. This review attempts to summarize our current state of knowledge regarding the role of ZFPs in the molecular regulation of skeletogenesis.

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.

The Zinc Finger Transcription Factor Transforming Growth Factor β-Inducible Early Gene-1 Confers Myeloid-specific Activation of the Leukocyte Integrin CD11d Promoter

CD11d encodes the alpha(D) subunit for a leukocyte integrin that is expressed on myeloid cells. In this study we show that the -100 to -20 region of the CD11d promoter confers myeloid-specific activation of the CD11d promoter. Transforming growth factor beta-inducible early gene-1 (TIEG1) was isolated in a yeast one-hybrid screen using the -100 to -20 region of the CD11d promoter as bait. Purified GST.TIEG1 protein was able to bind within the -61 to -45 region that overlaps a shorter binding site for Sp1. Transient overexpression of TIEG1 activated the CD11d promoter specifically in myeloid cells, whereas, down-regulation of TIEG1 with small interfering TIEG1 RNA also down-regulated expression of CD11d. In vivo, TIEG1 does not physically interact with Sp1. Cotransfection and electrophoretic mobility shift analyses of TIEG1, Sp1, and Sp3 revealed that TIEG1 competes with these Sp proteins for binding to overlapping sites in the CD11d promoter. Although TIEG1 and Sp1 are ubiquitously expressed in myeloid and non-myeloid cells, chromatin immunoprecipitation assays revealed differential occupancy of the CD11d promoter by these factors. In undifferentiated myeloid and non-myeloid cells, occupancy of the CD11d promoter by TIEG1 is similar. Upon differentiation of myeloid cells and subsequent up-regulation of CD11d expression, TIEG1 occupancy increases. In contrast, occupancy by TIEG1 remains low in non-myeloid cells exposed to phorbol ester. We propose that up-regulation of CD11d expression following differentiation of myeloid cells is mediated through increased binding of TIEG1 binding to the CD11d promoter.

Regulation of Fgf10 gene expression in the prostate: identification of transforming growth factor-beta1 and promoter elements

Fibroblast growth factor 10 (FGF10) is a mesenchymal paracrine-acting factor that plays a key role in the organogenesis of the prostate, and Fgf10 transcripts exhibit a highly restricted expression pattern within prostatic mesenchyme. To study the regulation of Fgf10 we have used organ rudiments grown in vitro as well as a primary stromal cell system derived from the ventral mesenchymal pad (VMP), a condensed area of mesenchyme known to induce prostatic organogenesis. Characterization of VMP cells (VMPCs) showed that they retained expression of AR as well as transcripts for FGF10 and TGFbeta1, -2, and -3. We propose that VMPCs are a good model of specialized mesenchyme involved in prostatic organogenesis and are distinct from general urogenital sinus mesenchyme/stroma. Treatment of VMPCs with TGFbeta1 resulted in a rapid and transient decrease in Fgf10 transcript levels, which were reduced 9-fold at 3 h. TGFbeta1 also inhibited Fgf10 expression in VMP organ rudiments grown in vitro. To further analyze Fgf10 regulation, 6 kb of mouse genomic sequence 5' to the translation start site was characterized by promoter analysis. Deletion analysis of the Fgf10 promoter in VMPCs identified a region of the promoter that mediated a significant proportion of promoter activity as well as mediating promoter down-regulation by TGFbeta1. This element was located between nucleotides -182 and -172 and contained a consensus Sp1 binding site. Taken together, our data suggest that TGFbeta1 is a regulator of Fgf10 expression in prostatic mesenchyme and that a proximal element within the Fgf10 promoter plays an important role in its regulation and expression.

Gene structure and evolution of Tieg3, a new member of the Tieg family of proteins

TGF beta-inducible immediate early gene, Tieg, belongs to the superfamily of Sp1-like transcription factors containing three C(2)H(2)-zinc finger DNA binding motifs close to the C-terminus. So far, Tieg1 and Tieg2 have been identified in human and mouse. We identified Tieg3, a new member of the Tieg protein family by screening a mouse cDNA library. Tieg3 has almost all the known features of the Tieg protein family: it shares a highly conserved C(2)H(2) zinc finger DNA binding domain and is 96% identical to Tieg2 and 86% to Tieg1, respectively. In addition, the three repression domains at the N-terminus, R1, R2 and R3 are conserved in all the Tiegs. Similar to Tieg1 and Tieg2, Tieg3 mRNA is up-regulated in response to TGF beta 1 treatment and can perform the Sp1 sites mediated repression of transcription. A 4 kilobase (kb) long transcript of mouse Tieg3 can be detected using Northern-blot analysis. The gene of mouse Tieg3 contains four exons. Due to the amino acid sequence similarity, mouse Tieg2 is regarded as an orthologue of human Tieg2. However, the mouse Tieg3 gene is localized in a conserved segment on mouse chromosome 12 corresponding to human Tieg2 on chromosome 2 with the same gene order. An interesting explanation for this apparent contradiction might be a homologous recombination leading to loci exchange between the mouse Tieg3 and Tieg2.

Fundamentals of transcription factors and their impact on pancreatic development and cancer

Transcription factors are proteins that regulate gene expression by modulating the synthesis of messenger RNA. Since this process, known as gene transcription, is often the dominant control point in the production of many proteins, transcription factors are key regulators of numerous cellular functions, including secretion, proliferation, differentiation, and apoptosis. Most transcription factors are also the final effectors of signaling pathways that transduce signals from the cell membrane to the nucleus. Therefore alterations in the activity or expression of some transcription factors have a significant impact on the biology of human cells and may lead to the development of diseases. In this article we review this field of research with a particular emphasis on the role of transcription factors in pancreatic development and cancer.

TIEG1 facilitates transforming growth factor-beta-mediated apoptosis in the oligodendroglial cell line OLI-neu

Transforming growth factor-beta (TGF-beta) plays an important role during the period of developmental cell death in the nervous system. Using the oligodendroglial precursor cell line OLI-neu, we have previously established an in vitro system to analyze TGF-beta-mediated cell death on the molecular level. We could show that the Krüppel-like Zn-finger transcription factor TIEG1 was up-regulated after TGF-beta stimulation of OLI-neu cells and mimicked TGF-beta effects in these cells; i.e., overexpression of TIEG1 in OLI-neu cells induced apoptosis as shown by apoptosis ELISA, DNA fragmentation, and caspases-3 activation. The apoptotic pathway seemed to be initiated by repressing the expression of the antiapoptotic protein Bcl-XL. In contrast, the reporter activity of a SMAD consensus promoter was induced, whereas the promoter activity of the inhibitory SMAD7 was reduced, suggesting that SMAD-dependent TGF-beta responses, such as TGF-beta-induced apoptosis, are enhanced in the presence of TIEG1.

15-hydroxy-eicosatetraenoic acid arrests growth of colorectal cancer cells via a peroxisome proliferator-activated receptor gamma-dependent pathway

Peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits cell growth via promoting apoptosis. Human colorectal cancer tissues had abundant PPARgamma but the incidence of apoptosis was very low, suggesting a defect in the PPARgamma pathway. Here, we found that 15-hydroxy-eicosatetraenoic acid (15S-HETE), an endogenous ligand for PPARgamma, was significantly decreased in the serum of patients with colorectal cancer. Treatment of colon cancer cells with 15S-HETE inhibited cell proliferation and induced apoptosis, which was preceded by an increase in TGF-beta-inducible early gene (TIEG) and a decrease in Bcl-2. The action of 15S-HETE could be blocked when PPARgamma was suppressed. Overexpression of Bcl-2 prevented the apoptosis. The levels of TIEG and 15-lipoxygenase (15-LOX), the enzyme responsible for 15S-HETE production, was decreased in colorectal cancer. Therefore, colorectal cancer is associated with decreased 15S-HETE. Treatment of colon cancer cells with 15S-HETE inhibits cell proliferation and induces apoptosis in a PPARgamma-dependent pathway involving augmentation of TIEG and reduction of Bcl-2 expression.

Cell-specific pituitary gene expression profiles after treatment with leukemia inhibitory factor reveal novel modulators for proopiomelanocortin expression

Leukemia inhibitory factor (LIF) mediates the hypothalamo-pituitary-adrenal stress response. Transgenic mice overexpressing LIF in the developing pituitary have altered pituitary differentiation with expansion of corticotropes, maintenance of Rathke's cleft cysts, and suppression of all other pituitary cell types. Affymetrix GeneChips were used to identify modulators of LIF effects in corticotrope (AtT-20) and somatolactotrope (GH(3)) cells. In addition to genes known to respond to LIF in corticotrope cells [e.g. suppressor of cytokine signaling-3 (SOCS-3), signal transducer and activator of transcription-3, SH2 domain-containing tyrosine phosphatase-1, and proopiomelanocortin (POMC)], corticotrope-specific changes were also observed for genes involved in glycolysis and gluconeogenesis, transcription factors, signaling molecules, and expressed sequence tags. Two transcription factors identified, CCAAT/enhancer-binding protein beta (C/EBPbeta) and glial cell-derived neurotrophic factor (GDNF)-inducible factor (GIF), dose-dependently induced expression of the rat POMC promoter when overexpressed in AtT-20 cells. LIF further induced POMC transcription with C/EBPbeta, but not with GIF. C/EBPbeta also induced expression of the SOCS-3 promoter that was further enhanced by cotreatment with LIF. However, GIF did not affect SOCS-3 expression. These results indicate that C/EBPbeta and GIF are downstream effectors of LIF corticotrope action. LIF also stimulates the expression of inhibitors of its actions, such as SOCS-3 and SH2 domain-containing tyrosine phosphatase-1. alpha(2)-HS-glycoprotein (AHSG)/fetuin, a secreted protein that antagonizes bone TGFbeta/bone morphogenic protein signaling, was induced by LIF in a signal transducer and activator of transcription-3-dependent fashion. Pretreatment with AHSG/fetuin blocked LIF-induced expression of the POMC promoter independently of SOCS-3. Thus, using GeneChips, C/EBPbeta and GIF have been identified as novel mediators and AHSG/fetuin as an inhibitor of LIF action in corticotropes.

Expression of alternatively spliced isoforms of human Sp7 in osteoblast-like cells

BACKGROUND

Osteogenic and chondrocytic differentiation involves a cascade of coordinated transcription factor gene expression that regulates proliferation and matrix protein formation in a defined temporo-spatial manner. Bone morphogenetic protein-2 induces expression of the murine Osterix/Specificity protein-7 (Sp7) transcription factor that is required for osteoblast differentiation and bone formation. Regulation of its expression may prove useful for mediating skeletal repair.

RESULTS

Sp7, the human homologue of the mouse Osterix gene, maps to 12q13.13, close to Sp1 and homeobox gene cluster-C. The first two exons of the 3-exon gene are alternatively spliced, encoding a 431-residue long protein isoform and an amino-terminus truncated 413-residue short protein isoform. The human Sp7 protein is a member of the Sp family having 78% identity with Sp1 in the three, Cys2-His2 type, DNA-binding zinc-fingers, but there is little homology elsewhere. The Sp7 mRNA was expressed in human foetal osteoblasts and craniofacial osteoblasts, chondrocytes and the osteosarcoma cell lines HOS and MG63, but was not detected in adult femoral osteoblasts. Generally, the expression of the short (or beta) protein isoform of Sp7 was much higher than the long (or alpha) protein isoform. No expression of either isoform was found in a panel of other cell types. However, in tissues, low levels of Sp7 were detected in testis, heart, brain, placenta, lung, pancreas, ovary and spleen.

CONCLUSIONS

Sp7 expression in humans is largely confined to osteoblasts and chondrocytes, both of which differentiate from the mesenchymal lineage. Of the two protein isoforms, the short isoform is most abundant.

Nitric oxide-mediated upregulation of the TGF-beta-inducible early response gene-1 (TIEG1) in human fibroblasts by mRNA stabilization independent of TGF-beta

Nitric oxide serves various roles in mammalian cells, including intracellular signaling and cell killing. To recognize the dynamic molecular changes in response to NO, microarray analysis was applied to human fibroblasts (IMR-90) exposed to sublethal levels of NO. Among the > 300 transcripts induced by NO, we focused on the mRNA encoded by the transforming growth factor-beta- (TGF-beta-) inducible early response 1 gene (TIEG1), which plays a pivotal role in TGF-beta-regulated cell growth control and apoptosis. Northern blotting analysis demonstrated that NO upregulates TIEG1 mRNA in a dose-dependent manner. Anti-TGF-beta antibodies prevented TIEG1 mRNA induction by TGF-beta, but not the induction by NO. Conversely, NO had no effect on the amounts of total TGF-beta or its active form in culture supernatants. However, the half-life of the TIEG1 transcript was strongly increased (6-fold) upon exposure of the cells to NO. Thus, NO upregulates TIEG1 mRNA by stabilization independently of TGF-beta. The TIEG1 mRNA now joins heme oxygenase-1 mRNA in displaying regulation by NO-mediated stabilization. It remains to be determined whether the same control mechanism operates on these and perhaps other messages.

Pancreatic carcinoma cell lines with SMAD4 inactivation show distinct expression responses to TGFB1

Transforming growth factor beta-1 (TGFB1)-induced gene expression was studied in five pancreatic carcinoma cell lines and one known TGFB1-sensitive cell line (HaCaT) by use of high-density filter-based cDNA microarrays representing over 4,000 human genes. The results indicate a complex cellular response to TGFB1 with 10% of the investigated genes showing altered expression after 3 or 48 hr of TGFB1 exposure. The tumor cell lines displayed a gradually inversed gene expression pattern, which correlated with reduced sensitivity to TGFB1, as compared to the HaCaT cell line. In the HaCaT cells, several proapoptotic genes showed increased expression in response to TGFB1, whereas the expression of antiapoptotic genes was decreased. In contrast, two pancreatic carcinoma cell lines, previously found to be growth stimulated by TGFB1, displayed an expression pattern opposite to that of these genes. Similarly, the expression of other functional groups of genes, such as cell cycle and transcription factor related genes, was almost completely reversed in these two tumor cell lines. Importantly, three of the five investigated pancreatic carcinoma cell lines responded to TGFB1, although they had SMAD4 inactivations, suggesting that the observed gene expression changes in these cell lines must be accomplished by SMAD-independent pathways.

H1t/GC-box and H1t/TE1 element are essential for promoter activity of the testis-specific histone H1t gene

The testis-specific linker histone H1t gene is transcribed exclusively in mid to late pachytene primary spermatocytes. Tissue-specific expression of the gene is mediated primarily through elements located within the proximal promoter. Previous work in transgenic animals identified a unique 40-base pair promoter element designated H1t/TE that is essential for spermatocyte-specific expression. The H1t/TE element contains three subelements designated TE2, GC-box, and TE1 based on in vitro footprinting and electrophoretic mobility shift assays. Because GC-box is a consensus site for binding of Sp transcription-factor family members, experiments were performed demonstrating that two Sp family members, Sp1 and Sp3, were present in testis cells from 9-day-old and adult rats and in pachytene primary spermatocytes and early spermatids. A 95- to 105-kDa form of Sp1 is most abundant in the tissues and cell lines examined, but a 60-kDa form of Sp1 is the most abundant species in spermatocytes and early spermatids. Further examination of Sp1 and Sp3 from adult testis, primary spermatocytes, and early spermatids showed that they can bind to the H1t/TE element. In order to determine the contributions of the subelements to H1t transcription, we mutated each of them in H1t promoter luciferase reporter vectors. Mutation of the GC-box and TE1 subelement reduced expression 77% and 49%, respectively, compared with the wild-type H1t promoter in transient expression assays in a testis GC-2spd cell line that was derived from germinal cells. These studies suggest that Sp transcription factors may be involved in transcription of the H1t gene and the GC-box and the TE1 subelement are required for activation of the H1t promoter.

Identification and characterization of a consensus DNA binding element for the zinc finger transcription factor TIEG/EGRalpha

TGFbeta-Inducible Early Gene (TIEG) and the alternatively-transcribed Early Growth Response Gene alpha (EGRalpha) share a Cys(2)His(2) three-zinc finger region with high homology to Sp1 within its zinc finger region. Three-zinc finger transcription factors bind to GC-rich sequences, with small variations in consensus sequence between subfamilies. In this work, a consensus sequence was identified for TIEG/EGRalpha by expressing and purifying the zinc finger region of the protein, and using this to select a binding site from a random oligonucleotide library by iterative cycles of nitrocellulose filter binding and PCR. A fusion of the TIEG/EGRalpha with the VP16 activation domain supported transcription from this site when cloned in front of a heterologous promoter. Mutational analysis of the binding site identified a GT-rich core (5'-GGTGTG-3') that was necessary for binding, with mutations outside of this region causing only a small to moderate decrease in binding.

Sp1 and Sp3 activate the testis-specific histone H1t promoter through the H1t/GC-box

The testis-specific linker histone H1t gene is transcribed exclusively in mid to late pachytene primary spermatocytes. Tissue specific expression of the gene is mediated in large part through elements located within the proximal promoter. Previous work in transgenic animals showed that a unique 40 bp promoter element designated H1t/TE is essential for spermatocyte-specific expression. The H1t/TE element contains a GC-box, which is a perfect consensus binding site for members of the Sp family of transcription factors. We have shown that Sp1 and Sp3 are present in testis cells from 9-day-old and adult rats and in pachytene primary spermatocytes and early spermatids and that they can bind to the H1t/GC-box. Mutagenesis of the GC-box reduced H1t promoter activity. Furthermore, a CpG dinucleotide within the GC-box was totally unmethylated in rat testis primary spermatocytes where the gene is transcribed but it was methylated in liver where the gene is silenced. These previous studies supported the importance of the GC-box and suggested that Sp transcription factors contribute to expression of the H1t gene. In this study, we show that co-transfection of Sp1 and Sp3 expression vectors leads to an upregulation of histone H1t promoter activity in several cell lines including testis GC-2spd cells. However, very low H1t promoter activity is seen in GC-2spd cells grown at 39 degrees C, which correlates with lower levels of Sp1 and Sp3 in these cells grown at this elevated temperature. Upregulation of the H1t promoter by Sp1 and Sp3 was also seen in cotransfected NIH3T3 and C127I cell lines. On the other hand, co-transfection of the Sp1 and Sp3 expression vectors does not lead to upregulation of activity of the cell-cycle dependent histone H1d promoter.

Genomic structure and functional characterisation of the promoters of human and mouse nogo/rtn4

The reticulon-family member Nogo-A is a potent neurite growth inhibitory protein in vitro and may play a role in the restriction of axonal regeneration after injury and of structural plasticity in the CNS of higher vertebrates. Of the three major isoforms of Nogo, Nogo-A is mostly expressed in the brain, Nogo-B is found in a ubiquitous pattern, and Nogo-C is most highly expressed in muscle. Seven additional splice-variants derived both from differential splicing and differential promoter usage have been identified. Analysis of the TATA-less Nogo-A/B promoter (P1) shows that conserved GC-boxes and a CCAAT-box within the first 500bp upstream of the transcription start are responsible for its regulation. No major differences in the methylation status of the P1 CpG-island in tissues expressing or not expressing Nogo-A/B could be detected, suggesting that silencer elements are involved in the regulation. The specific expression pattern of Nogo-A/B is due to differential splicing. The basal Nogo-C promoter (P2) is regulated by a proximal and a distal element. The 5'UTR of Nogo-C harbours a negative control element. These data may help to identify factors that can modulate Nogo transcription, thus offering an alternative approach for Nogo neutralisation.

Microarray analysis of bone morphogenetic protein, transforming growth factor beta, and activin early response genes during osteoblastic cell differentiation

Bone morphogenetic protein (BMP) 2, a member of the transforming growth factor (TGF) beta family, is a potent regulator of osteoblast differentiation. In addition, both TGF-beta and activin A can either induce bone formation or inhibit bone formation depending on cell type and differentiation status. Although much is known about the receptors and intracellular second messengers involved in the action of TGF-beta family members, little is known about how selectivity in the biological response of individual family members is controlled. In this study, we have investigated selective gene induction by BMP-2, TGF-beta1 and activin A in relation to their ability to control differentiation of mouse mesenchymal precursor cells C2C12 into osteoblastic cells. TGF-beta1 can inhibit BMP-2-induced differentiation of these cells, whereas activin A was found to be without morphogenetic effect. Using a gene expression microarray approach covering 8636 sequences, we have identified a total of 57 established genes and expressed sequence tags (ESTs) that were either up-regulated or down-regulated 2 h after treatment with at least one of these three stimuli. With respect to the established genes, 15 new target genes for TGF-beta family members thus were identified. Furthermore, a set of transcripts was identified, which was oppositely regulated by TGF-beta1 and BMP-2. Based on the inverse biological effects of TGF-beta1 and BMP-2 on C2C12 cells, these genes are important candidates for controlling the process of growth factor-induced osteoblast differentiation.

The zinc finger transcription factor Zfp60 is a negative regulator of cartilage differentiation

The differentiation of many mesenchyme-derived cells, including cells that form bone and cartilage, is regulated at the level of gene transcription, but many of the factors involved in this regulation remain to be identified. In this study, a modified RNA fingerprinting technique was used to identify the KRAB domain zinc finger transcription factor Zfp60 as a candidate regulator of cell differentiation in mouse calvaria primary cultures. The highest expression of Zfp60 mRNA in vivo was found between embryonic day 11 (E11) and E15 during mouse embryonic development, coinciding with stages of active organ formation. The expression of Zfp60 mRNA and protein was analyzed further in mouse embryos during skeletal development. The most prominent expression was found in prehypertrophic chondrocytes, where it coincides with the expression of key regulators of chondrocyte maturation, Indian hedgehog (Ihh), and the parathyroid hormone-related peptide (PTHrP) receptor. Zfp60 mRNA was also found transiently expressed during chondrogenesis of C1 cells in vitro, preceding collagen type X expression and cellular hypertrophy. Overexpression of Zfp60 inhibited cartilage differentiation in the chondrogenic ATDC5 cell line. These results suggest a role for Zfp60 as a negative regulator of gene transcription, specifically during the development and/or differentiation of chondrocytes.

Absence of mutations in the transforming growth factor-beta inducible early gene 1, TIEG1, in pancreatic cancer

Pancreatic cancers frequently have defects in components of the transforming growth factor-beta (TGF-beta) signaling pathway. TIEG1 (TGF-beta inducible early gene) is a recently characterized transcription factor regulated by TGF-beta that induces apoptosis when overexpressed in pancreatic adenocarcinoma cell lines. Alterations on chromosome 8q, where TIEG1 is located, are also relatively frequent in pancreatic cancers. To determine if TIEG1 may be involved in the tumorigenesis of pancreatic cancer, we performed mutational screening of this gene in 22 pancreatic cancer cell lines. No sequence alterations were observed. Reverse transcription-polymerase chain reaction analysis was also performed to rule out the possibility that the expression of the gene is altered by genetic events other than mutation. Likewise, no alterations in expression were found. Thus, an essential role of TIEG1 in pancreatic cancer can be excluded.

Transcriptional regulation of Smad2 is required for enhancement of TGF?/Smad signaling by TGF? inducible early gene

TGFbeta inducible early gene (TIEG) is a novel Krüppel-like transcriptional repressor that was recently shown to increase the activity of the TGFbeta/Smad signal transduction pathway by relieving negative feedback through repression of the inhibitory Smad7. Interestingly, while Smad7 is required for maximal enhancement of TGFbeta/Smad signaling, we observe that TIEG is still capable of increasing Smad pathway activity in the absence of Smad7. Furthermore, while Smad7 is known to block both TGFbeta and bone morphogenetic protein (BMP) signaling, we observe that TIEG specifically enhances only the TGFbeta pathway. Similarly, while both TIEG and the related Krüppel-like factor, FKLF2, repress Smad7 transcription, only TIEG is capable of enhancing Smad signaling. In order to identify additional regulatory targets of TIEG important for this enhancement of the Smad pathway activity, we performed microarray analysis and identified Smad2 as a TIEG target gene. We now show evidence that TIEG increases transcription of the Smad2 gene but not the Smad3 or Smad4 genes. Furthermore, while the TGFbeta/Smad pathway remains intact in Smad2 null cells, TIEG enhancement of Smad signaling is dramatically reduced. Thus we propose a new model whereby TIEG enhances Smad signaling by a dual mechanism involving both the repression of the inhibitory Smad7 as well as the activation of Smad2.

Modulation of transforming growth factor beta (TGFbeta)/Smad transcriptional responses through targeted degradation of TGFbeta-inducible early gene-1 by human seven in absentia homologue

Transforming growth factor beta (TGFbeta)-inducible early gene-1 (TIEG1) is a Krüppel-like transcription factor that is rapidly induced upon TGFbeta treatment. TIEG1 promotes TGFbeta/Smad signaling by down-regulating negative feedback through the inhibitory Smad7. In this report, we describe the identification of an E3 ubiquitin ligase, Seven in Absentia homologue-1 (SIAH1), as a TIEG1-interacting protein. We show that TIEG1 and SIAH1 interact through an amino-terminal domain of TIEG1. Co-expression of SIAH1 results in proteasomal degradation of TIEG1 but not of the related factor TIEG2. Importantly, co-expression of SIAH1 completely reverses repression of Smad7 promoter activity by TIEG1. Furthermore, overexpression of a dominant negative SIAH1 stabilizes TIEG1 and synergizes with TIEG1 to enhance TGFbeta/Smad-dependent transcriptional activation. These findings suggest a novel mechanism whereby the ability of TGFbeta to modulate gene transcription may be regulated by proteasomal degradation of the downstream effector TIEG1 through the SIAH pathway. In this manner, turnover of TIEG1 may serve to limit the duration and/or magnitude of TGFbeta responses.

TGFbeta inducible early gene enhances TGFbeta/Smad-dependent transcriptional responses

TGFbeta inducible early gene (TIEG) encodes a three zinc-finger Krüppel-like transcription factor whose overexpression has been shown to mimic the effects of TGFbeta in human osteosarcoma and pancreatic carcinoma cells. In order to investigate a potential role of TIEG in the TGFbeta signal transduction pathway, we studied its impact on a Smad binding element (SBE) reporter which is known to be regulated by TGFbeta through the R-Smad proteins. We demonstrate that TIEG overexpression enhances TGFbeta induction of SBE reporter activity. TIEG overexpression also enhances induction of the endogenous TGFbeta regulated genes p21 and PAI-1. The ability of TIEG to enhance TGFbeta actions is Smad dependent since TIEG has no effect on SBE transcription in the absence of Smad4 expression or when an inhibitory Smad protein, Smad7, is overexpressed. Furthermore, TIEG overexpression enhances TGFbeta induced Smad2 phosphorylation. Lastly, TIEG appears to function by binding to and thereby repressing a specific element in the proximal promoter of the inhibitory Smad7 gene. In conclusion, these results describe a novel mechanism for the potentiation of TGFbeta/Smad signaling via repression of the inhibitory Smad7 gene by TIEG.

Functional analysis of zinc finger proteins that bind to the silencer element in the glutathione transferase P gene

Glutathione transferase P (GST-P) gene expression is repressed in normal rats but markedly promoted during the early stage of chemical hepatocarcinogenesis. We have previously identified a silencer region in this gene promoter. The silencer is composed of several cis-elements to which at least three proteins (Silencer factor-A, -B, and -C: SF-A, SF-B, and SF-C) are known to bind. We cloned and characterized the nuclear factor 1 family and the CCAAT/enhancer-binding protein family as SF-A and SF-B, respectively. Recently, zinc finger proteins as candidates for SF-C, which binds to GST-P silencer 2 (GPS2), were isolated. These proteins include four Krüppel-like proteins (BTEB2, EZF, LKLF, and TIEG1) and other factors containing multiple zinc finger motifs (TFIIIA and MZFP). In the present study, we found that the zinc finger proteins showed the same DNA-binding affinities to GPS2. Moreover, transfection analyses revealed that BTEB2, EZF, and TIEGI repressed the GST-P promoter activity. Therefore, these three factors might contribute to the repression of the GST-P gene expression in normal rat liver.

XSPR-1 and XSPR-2, novel Sp1 related zinc finger containing genes, are dynamically expressed during Xenopus embryogenesis

Proteins related to the human transcription factor Sp1 are characterized by the presence of a highly conserved zinc finger domain consisting of three C2H2 type zinc fingers. Here we describe two Xenopus laevis cDNAs, which encode novel Sp1-related C2H2 type zinc finger transcription factors named XSPR-1 and XSPR-2. Structurally, XSPR-1 and XSPR-2 are closely related to the murine Sp5, which interacts genetically with Brachyury (Dev. Biol. 227 (2000) 358). XSPR-1 and XSPR-2 are expressed in broad and dynamic patterns during early development. Starting at gastrulation, XSPR-1 transcripts are restricted to the non-involuting marginal zone, and, at later stages, to the neuroectoderm, forebrain, otic vesicles and the midbrain/hindbrain boundary. In contrast, XSPR-2 expression is found predominantly within the presumptive mesoderm during gastrulation. At tailbud and tadpole stages, XSPR-2 is expressed exclusively in the tip of the tail.

Brief exposure to high-dose transforming growth factor-beta1 enhances periosteal chondrogenesis in vitro: a preliminary report

BACKGROUND

Articular cartilage has limited potential for repair. There have been various attempts aimed at improving the repair process in articular cartilage. Transforming growth factor-beta1 (TGF-beta1) has a stimulatory effect on chondrogenesis in periosteal explants. The purpose of the present study was to determine the effect of brief exposures (i.e., thirty and sixty minutes) of high concentrations of TGF-beta1 on periosteal chondrogenesis.

METHODS

Five hundred and seventy-three periosteal explants were harvested from forty-six two-month-old male New Zealand White rabbits. Explants were exposed to 50 or 100 ng/mL of TGF-beta1 for thirty or sixty minutes. The amount of cartilage formed was then determined with use of a standardized six-week agarose culture assay.

RESULTS

There was a significant increase in the amount of cartilage formation (p < 0.01), Type-II collagen content (p < 0.05), and sulfate incorporation (p < 0.0001) in explants treated with TGF-beta1. Maximal stimulation occurred following exposure to 100 ng/mL of TGF-beta1 for thirty minutes. There was also an increase in chondrocyte proliferation as measured by [ (3) H-] thymidine incorporation on day 5 of culture (p < 0.049).

CONCLUSIONS

The findings of this study indicate that exposure to TGF-beta1 has a stimulatory effect on periosteal chondrogenesis. This stimulatory effect is observed even with a very brief exposure time of thirty minutes.

CLINICAL RELEVANCE

A possible clinical application of these findings is exposure of periosteal grafts that are currently utilized clinically to resurface articular defects to TGF-beta1 during the short time between graft procurement and implantation into the joint. This may obviate the need for intra-articular administration of TGF-beta1 and may enhance the ultimate graft incorporation and quality of cartilage repair.

Tissue culture models for studies of hormone and vitamin action in bone cells

Osteoporosis is a major health care concern and levies a serious financial burden on the world health care system. For this reason, many physicians and scientists are engaged in research to better understand and treat this disease. To this end, numerous in vitro bone cell models have been developed to explore the cellular and molecular mechanisms of skeletal biology and for the identification and characterization of new drug targets and therapies. In this chapter, we review many of these cellular models as tools to study the hormonal regulation of bone metabolism. In particular, we pay special attention to new human bone cell models, since these have the greatest relevance to osteoporosis research and drug discovery. These new models include (1) the use of peripheral blood mononuclear cells as progenitors of osteoclasts and primary cultures of mesenchymal stem cells as precursors of osteoblasts; (2) the development of conditionally immortalized preosteoclastic and osteoblastic cell lines using temperature-sensitive large T-antigens; and (3) the establishment of the first osteocytic cell lines. Thus, we now have at our disposal many good in vitro models to investigate the regulation of bone resorption and formation by hormones, vitamins and drugs. These models should accelerate our understanding of bone physiology and pathophysiology as well as our ability to develop important new therapies to prevent and treat skeletal diseases.

A conserved alpha-helical motif mediates the interaction of Sp1-like transcriptional repressors with the corepressor mSin3A

Sp1-like proteins are defined by three highly homologous C(2)H(2) zinc finger motifs that bind GC-rich sequences found in the promoters of a large number of genes essential for mammalian cell homeostasis. Here we report that TIEG2, a transforming growth factor beta-inducible Sp1-like protein with antiproliferative functions, represses transcription through recruitment of the mSin3A-histone deacetylase complex. The interaction of TIEG2 with mSin3A is mediated by an alpha-helical repression motif (alpha-HRM) located within the repression domain (R1) of TIEG2. This alpha-HRM specifically associates with the second paired amphipathic helix (PAH2) domain of mSin3A. Mutations in the TIEG2 alpha-HRM domain that disrupt its helical structure abolish its ability to both bind mSin3A and repress transcription. Interestingly, the alpha-HRM is conserved in both the TIEG (TIEG1 and TIEG2) and BTEB (BTEB1, BTEB3, and BTEB4) subfamilies of Sp1-like proteins. The alpha-HRM from these proteins also mediates direct interaction with mSin3A and represses transcription. Surprisingly, we found that the alpha-HRM of the Sp1-like proteins characterized here exhibits structural and functional resemblance to the Sin3A-interacting domain previously described for the basic helix-loop-helix protein Mad1. Thus, our study defines a mechanism of transcriptional repression via the interactions of the alpha-HRM with the Sin3-histone deacetylase complex that is utilized by at least five Sp1-like transcriptional factors. More importantly, we demonstrate that a helical repression motif which mediates Sin3 interaction is not an exclusive structural and functional characteristic of the Mad1 subfamily but rather has a wider functional impact on transcriptional repression than previously demonstrated.

Sp1 and krüppel-like factor family of transcription factors in cell growth regulation and cancer

The Sp/KLF family contains at least twenty identified members which include Sp1-4 and numerous krüppel-like factors. Members of the family bind with varying affinities to sequences designated as 'Sp1 sites' (e.g., GC-boxes, CACCC-boxes, and basic transcription elements). Family members have different transcriptional properties and can modulate each other's activity by a variety of mechanisms. Since cells can express multiple family members, Sp/KLF factors are likely to make up a transcriptional network through which gene expression can be fine-tuned. 'Sp1 site'-dependent transcription can be growth-regulated, and the activity, expression, and/or post-translational modification of multiple family members is altered with cell growth. Furthermore, Sp/KLF factors are involved in many growth-related signal transduction pathways and their overexpression can have positive or negative effects on proliferation. In addition to growth control, Sp/KLF factors have been implicated in apoptosis and angiogenesis; thus, the family is involved in several aspects of tumorigenesis. Consistent with a role in cancer, Sp/KLF factors interact with oncogenes and tumor suppressors, they can be oncogenic themselves, and altered expression of family members has been detected in tumors. Effects of changes in Sp/KLF factors are context-dependent and can appear contradictory. Since these factors act within a network, this diversity of effects may arise from differences in the expression profile of family members in various cells. Thus, it is likely that the properties of the overall network of Sp/KLF factors play a determining role in regulation of cell growth and tumor progression.

Dopamine receptor regulating factor, DRRF: a zinc finger transcription factor

Dopamine receptor genes are under complex transcription control, determining their unique regional distribution in the brain. We describe here a zinc finger type transcription factor, designated dopamine receptor regulating factor (DRRF), which binds to GC and GT boxes in the D1A and D2 dopamine receptor promoters and effectively displaces Sp1 and Sp3 from these sequences. Consequently, DRRF can modulate the activity of these dopamine receptor promoters. Highest DRRF mRNA levels are found in brain with a specific regional distribution including olfactory bulb and tubercle, nucleus accumbens, striatum, hippocampus, amygdala, and frontal cortex. Many of these brain regions also express abundant levels of various dopamine receptors. In vivo, DRRF itself can be regulated by manipulations of dopaminergic transmission. Mice treated with drugs that increase extracellular striatal dopamine levels (cocaine), block dopamine receptors (haloperidol), or destroy dopamine terminals (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) show significant alterations in DRRF mRNA. The latter observations provide a basis for dopamine receptor regulation after these manipulations. We conclude that DRRF is important for modulating dopaminergic transmission in the brain.

Characterization of the Promoter Region of the Human Peroxisomal Multifunctional Enzyme Type 2 Gene

Peroxisomal multifunctional enzyme type 2 (perMFE-2) catalyzes conversion of (24E)-3alpha,7alpha, 12alpha-trihydroxy-5beta-cholest-24-enoyl-CoA to (24-keto)-3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoyl-CoA, which are physiological intermediates in cholic acid synthesis. In contrast to long chain fatty acid oxidizing enzymes clofibrate does not induce peroxisomal enzymes metabolizing bile acid intermediates. We proposed the existence of PPAR-independent regulation of cholesterol side chain oxidation in the process of bile acid synthesis. In the present study, we characterized the promoter region of the human perMFE-2 gene. The promoter contains the Sp1/AP2 binding site (-151/-142) within 197 base pairs upstream of the translation start site. Mutation of the Sp1/AP2 binding site decreases the promoter activity. Analysis by the luciferase assay revealed that the activity of the promoter region is strong in HepG2 and HeLa cell lines, although the activity in HepG2 cells was five- to sixfold higher than that in HeLa cells. Transient transfection assays have confirmed that AP2alpha and AP2gamma were able to transactivate the perMFE-2 promoter/luciferase chimeric gene. Cotransfections with Sp1 expression plasmid decreased the promoter activity. We suggest that perMFE-2 promoter activity is the result of both the abundance of AP2 and Sp1 family members and their relative ratios.

Functional activity of HERV-K-T47D-related long terminal repeats

The human genome contains a family of endogenous retroviruses, HERV-K(HML-4), that comprises the full-length provirus HERV-K-T47D, five related elements, and hundreds of solitary long terminal repeats (LTRs). We here show that HERV-K-T47D-related LTRs are dispersed over all human chromosomes and have arisen after the divergence of Old and New World monkeys. By screening a cDNA library derived from the human mammary carcinoma cell line T47D with a HERV-K-T47D LTR probe, we isolated several clones containing LTR/cellular gene chimeras and assessed the transcriptional activity of these LTRs in transient transfection experiments. All LTRs were able to drive the expression of a reporter gene, thereby displaying distinct activities in different cell lines. We found that sequences located downstream of the LTR-U3 region modulate the level of gene expression. Based on the impact of the R region we distinguished between three different LTR types; the activity of type I LTRs was enhanced in the presence of the LTR-R region in all cell lines tested, whereas a type II LTR was downregulated. Type III LTRs are characterized by lacking or having a varying influence of the R region that was dependent on the cell line used. Finally, our results attribute to LTR-U5-gag sequences a role in determining LTR activity.

Identification of KLF13 and KLF14 (SP6), novel members of the SP/XKLF transcription factor family

Using the sequence of the SP1 zinc-finger DNA-binding domain as a probe to screen a mouse EST database, we identified two novel members of the SP/XKLF transcription factor family, KLF13 and KLF14. The mouse Klf13 cDNA (1310 bp in length) contains a single open reading frame of 288 amino acids with a DNA-binding domain closely related to that of the human RFLAT-1 protein and a putative transactivator N-terminal domain rich in proline and alanine residues. The mouse Klf13 gene seems to be the homologue of the human RFLAT1 gene. The mouse Klf14 sequence is homologous to a human genomic sequence from chromosome 17 that is believed to code for a protein with three zinc fingers at the end of its C-terminal domain. Using reverse transcription-polymerase chain reaction, we showed ubiquitous expression of Klf13 and Klf14 in adult mice. A third member of this family was also identified in a human EST database; this sequence was found to be identical to KLF11 (TIEG2), recently identified by Cook et al. (1998, J. Biol. Chem. 273: 25929-25936). The corresponding mouse cDNA was isolated and sequenced. The three genes were localized in the human and the rat: chromosomes 15 (human KLF13), 17q21.3-q22 (human KLF14; HGMW-approved symbol SP6), and 2p25 (human KLF11) and chromosomes 1q31-q32 (rat Klf13), 10q31-q32.1 (rat Klf14) (SP6), and 6q16-q21 (rat Klf11).

Overexpression of a nuclear protein, TIEG, mimics transforming growth factor-beta action in human osteoblast cells

Although transforming growth factor-beta (TGF-beta) is a growth factor with many known regulatory activities in many different cell types, its intracellular signaling pathway is still not fully understood. A TGF-beta-inducible early gene (TIEG) was discovered and shown by this laboratory to be a 3-zinc finger transcription factor family member; its expression is rapidly induced in cells treated with TGF-beta. To ascertain whether TIEG plays a major role in the TGF-beta pathway, human osteosarcoma MG-63 cells were stably transfected either with an expression vector containing a TIEG cDNA or with the vector alone. Clones that contain only the vector express normal levels of TIEG mRNA and protein and display the same patterns of gene expression and levels of cell proliferation as the nontransfected, non-TGF-beta-treated parental cells. However, transfected cells that overexpress TIEG mRNA and protein (TIEG-6 and TIEG-7) display changes that mimic those of MG-63 cells treated with TGF-beta, i.e. increased alkaline phosphatase activity, decreased levels of osteocalcin mRNA and protein, and decreased cell proliferation. The degree of these changes correlated with the level of TIEG expressed in the cell lines. TGF-beta treatment of the overexpressed cells showed no added effects. These findings and other published reports support a primary role of TIEG as a transcription factor in the TGF-beta signaling pathway.

A set of proteins interacting with transcription factor Sp1 identified in a two-hybrid screening

The two-hybrid system was used to isolate cDNA clones encoding polypeptides that interact with the N-terminal region (activation domains A, B and C) of the Sp1 transcription factor. Among the 65 collected clones, 43 contained cDNA fragments with open reading frames. They corresponded to 13 genes encoding proteins of known function and to 15 genes, the proteins of which have no known function. Six overlapping cDNA clones corresponded to the Hsc70 protein. Host cell factor (HCF-1) and the KIAA0461 gene (encoding a putative Zn-finger protein of unknown function) were both identified through the isolation of three overlapping cDNA clones. Two cDNA fragments encoding the same region of the SREBP-2 transcription factor were independently selected and two overlapping cDNA clones corresponded to the splicing factor SF3A120. Two different cDNA clones encoded the N- and C-terminal region of the Oct-1 transcription factor. Transcription factors Elf-1 and TIEG, as well as HSph2, the putative human homologue of a murine polyhomeotic gene, were each represented by a single clone. Noticeably, for the four identified transcription factors, the DNA-binding domain was excluded from the selected polypeptides. In vitro binding of the selected polypeptides to the Sp1 protein was demonstrated for the four transcription factors and for the SF3A120, Hsc70, HCF-1, HSph2 and pKIAA0461(245) proteins. Four other cDNA clones encoding polypeptides of unknown function were tested in the in vitro binding assay. All four polypeptides were found to interact with Sp1 in this assay.

Cytokine-specific induction of the TGF-beta inducible early gene (TIEG): regulation by specific members of the TGF-beta family

Select members of the TGF-beta family of cytokines play key regulatory roles in skeletal development, structure, and turnover. This laboratory has previously reported that TGF-beta treatment of immortalized normal human fetal osteoblast (hFOB) cells results in the rapid induction of the mRNA levels of a TGF-beta inducible early gene (TIEG) followed by changes in cell proliferation and bone matrix protein production. Previous studies have also shown that nonmembers of the TGF-beta superfamily showed little or no induction of TIEG mRNA. This article further addresses the cytokine specificity of this TIEG induction by examining whether activin and select bone morphogenetic proteins, (BMP-2, BMP-4, and BMP-6), which are representative of different subfamilies of this superfamily, also induce the expression of TIEG in hFOB cells. However, TGF-beta remained the most potent of these cytokines, inducing TIEG mRNA steady-state levels at 0.1 ng/ml, with a maximum induction of 24-fold at 2.0 ng/ml. The BMP-2 (16-fold), BMP-4 (4-fold), and activin (1-3-fold) also induced TIEG mRNA levels, but at reduced degrees compared to TGF-beta (24-fold), and only at much higher cytokine concentrations, e.g., 50-100 ng/ml, compared to 2 ng/ml for TGF-beta. BMP-6 showed no effect on TIEG mRNA levels. The TIEG protein levels generally correlated with the mRNA steady-state levels. As with TGF-beta, BMP-2 treatment of hFOB cells was shown by confocal microscopy to induce a rapid translocation of the TIEG protein to the nucleus. In summary, the relative potencies of these TGF-beta family members to induce TIEG expression generally follows the general osteoinductive capacity of these cytokines, with TGF-beta >>> BMP-2 > BMP-4 > activin >> BMP-6.

FKLF-2: a novel Krüppel-like transcriptional factor that activates globin and other erythroid lineage genes

FKLF-2, a novel Krüppel-type zinc finger protein, was cloned from murine yolk sac. The deduced polypeptide sequence of 289 amino acids has 3 contiguous zinc fingers at the near carboxyl-terminal end, an amino-terminal domain characterized by its high content of alanine and proline residues and a carboxyl-terminal domain rich in serine residues. By Northern blot hybridization, the human homologue of FKLF-2 is expressed in the bone marrow and striated muscles and not in 12 other human tissues analyzed. FKLF-2 is constitutively expressed in established cell lines with an erythroid phenotype, but it is inconsistently expressed in cell lines with myeloid or lymphoid phenotypes. The expression of FKLF-2 messenger RNA (mRNA) is up-regulated after induction of mouse erythroleukemia cells. In luciferase assays, FKLF-2 activates predominantly the γ, and to a lesser degree, the ɛ and β globin gene promoters. The activation of γ gene promoter does not depend on the presence of an HS2 enhancer. FKLF-2 activates the γ promoter predominantly by interacting with the γ CACCC box, and to a lesser degree through interaction with the TATA box or its surrounding DNA sequences. FKLF-2 also activated all the other erythroid specific promoters we tested (GATA-1, glycophorin B, ferrochelatase, porphobilinogen deaminase, and 5-aminolevulinate synthase). These results suggest that in addition to globin, FKLF-2 may be involved in activation of transcription of a wide range of genes in the cells of the erythroid lineage.

FKLF-2: a novel Krüppel-like transcriptional factor that activates globin and other erythroid lineage genes

FKLF-2, a novel Krüppel-type zinc finger protein, was cloned from murine yolk sac. The deduced polypeptide sequence of 289 amino acids has 3 contiguous zinc fingers at the near carboxyl-terminal end, an amino-terminal domain characterized by its high content of alanine and proline residues and a carboxyl-terminal domain rich in serine residues. By Northern blot hybridization, the human homologue of FKLF-2 is expressed in the bone marrow and striated muscles and not in 12 other human tissues analyzed. FKLF-2 is constitutively expressed in established cell lines with an erythroid phenotype, but it is inconsistently expressed in cell lines with myeloid or lymphoid phenotypes. The expression of FKLF-2 messenger RNA (mRNA) is up-regulated after induction of mouse erythroleukemia cells. In luciferase assays, FKLF-2 activates predominantly the γ, and to a lesser degree, the ɛ and β globin gene promoters. The activation of γ gene promoter does not depend on the presence of an HS2 enhancer. FKLF-2 activates the γ promoter predominantly by interacting with the γ CACCC box, and to a lesser degree through interaction with the TATA box or its surrounding DNA sequences. FKLF-2 also activated all the other erythroid specific promoters we tested (GATA-1, glycophorin B, ferrochelatase, porphobilinogen deaminase, and 5-aminolevulinate synthase). These results suggest that in addition to globin, FKLF-2 may be involved in activation of transcription of a wide range of genes in the cells of the erythroid lineage.

TIEG proteins join the Smads as TGF-beta-regulated transcription factors that control pancreatic cell growth

The control of epithelial cell proliferation, differentiation, and apoptosis requires a balance between signaling and transcriptional regulation. Recent developments in pancreatic cell research have revealed that transforming growth factor-beta (TGF-beta) signaling is important for the regulation of each of these phenomena. More importantly, perturbations in this pathway are associated with pancreatic cancer. A chief example of these alterations is the mutation in the TGF-beta-regulated transcription factor Smad4/DPC4 that is found in a large percentage of pancreatic tumors. Surprisingly, studies on transcription factors have remained an underrepresented area of pancreatic research. However, the discovery of Smad4/DPC4 as a transcription factor fueled further studies aimed at characterizing transcription factors involved in normal and neoplastic pancreatic cell growth. Our laboratory recently described the existence of a novel family of zinc finger transcription factors, TGF-beta-inducible early-response gene (TIEG)1 and TIEG2, from the exocrine pancreas that, similarly to Smads, participate in the TGF-beta response and inhibit epithelial cell proliferation. This review therefore focuses on describing the structure and function of these two families of transcription factor proteins that are becoming key players in the regulation of pancreatic cell growth.

Direct action of naturally occurring estrogen metabolites on human osteoblastic cells

This article describes experiments that were performed to examine the direct action of estrogen metabolites on cultured human osteoblast cells. The human fetal osteoblastic cell line, hFOB/ER9, which expresses high levels of the estrogen receptor (ER) alpha, was used to examine the direct effects of 16alpha-hydroxyestrone (16alpha-OHE1) and 2-hydroxyestrone (2-OHE1) on osteoblast differentiation. The 16alpha-OHE1 caused a decrease in osteocalcin (OC) secretion to a maximum of 40% of control values (vehicle-treated cells) at 10(-7) M. Alkaline phosphatase (AP) activity was significantly induced at 10(-7) M 16alpha-OHE1 with greater than 500% of control at 10(-6) M 16alpha-OHE1. Finally, AP steady-state messenger RNA (mRNA) levels were increased within 24 h of 16alpha-OHE1 treatment. In contrast to 16alpha-OHE1, 2-OHE1 had no effects on the secretion of OC, AP activity, or AP gene expression. The 2-OHE1 also did not display any antiestrogen activity because treatment in combination with 17beta-estradiol (E2) and 16alpha-OHE1 had no significant effect on the reduction in OC secretion or induction of AP activity. Similar to E2, 16alpha-OHE1 stimulated the expression of an early response gene, a TGF-beta inducible early gene, designated TIEG, as early as 60 minutes after treatment, whereas treatment with 2-OHE1 displayed no effect. Support that the 16alpha-OHE1 regulation of these osteoblasts (OB) markers was mediated through the ER is shown by the fact that the estrogen antagonist ICI 182,780 abrogated these effects. These data suggest that is a potent estrogen agonist on human osteoblastic hOB/ER9 cells. In contrast, 2-OHE1 displayed no estrogenic or antiestrogenic activity in this human osteoblast cell model.