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At least 19 recordsLinked to original sources
PubMed1991

Mithramycin inhibits SP1 binding and selectively inhibits transcriptional activity of the dihydrofolate reductase gene in vitro and in vivo.

The promoter of the human dihydrofolate reductase (DHFR) gene contains two consensus binding sites for the DNA binding protein Sp1. DNAse protection and gel mobility shift assays demonstrate binding of recombinant Sp1 to both decanucleotide Sp1 binding sequences which are located 49 and 14 base pairs upstream of the transcription start site. The more distal of the two binding sites exhibits a somewhat higher affinity for Sp1. The G-C specific DNA binding drug, mithramycin, binds to both consensus sequences and prevents subsequent Sp1 binding. Promoter-dependent in vitro transcription of a DHFR template is selectively inhibited by mithramycin when compared to the human H2b histone gene. A similar effect is also noted in vivo. Mithramycin treatment of MCF-7 human breast carcinoma cells containing an amplified DHFR gene induces selective inhibition of DHFR transcription initiation, resulting in a decline in DHFR mRNA level and enzyme activity. This selective inhibition of DHFR expression suggests that it is possible to modulate the overexpression of the DHFR gene in methotrexate resistant cells.

Base Sequence↗
PubMed1994

Triplex formation by the human Ha-ras promoter inhibits Sp1 binding and in vitro transcription.

The central role of the ras oncogenes in the pathogenesis of a wide variety of human malignancies is well established. Toward developing specific transcriptional inhibitors of the human Ha-ras oncogene, we have designed oligonucleotides to target a region of the Ha-ras promoter (-8 to -28) which contains two of the three Sp1 binding sites essential for transcriptional activity. Gel mobility analysis and DNase I footprinting demonstrate that an oligonucleotide (HR21ap) forms a sequence-specific triple helix with its target site in an antiparallel orientation with respect to the purine-rich duplex strand through predominantly G*G:C triplets. Within the Ha-ras promoter, HR21ap binds exclusively to the proximal target Sp1 sites over a similar nontarget distal sequence which, like the target, contains a consensus Sp1 site. Protein binding assays demonstrate that triplex formation by HR21ap inhibits Sp1 binding to the Ha-ras promoter. Moreover, oligonucleotide-directed triplex formation arrests Ha-ras promoter-dependent transcription in vitro. The results presented here suggest that triplex formation by the Ha-ras promoter targeted oligonucleotide may provide a means to specifically inhibit transcription of this oncogene in vivo.

Base Sequence↗
PubMed2003

Sp1 inhibits proliferation and induces apoptosis in vascular smooth muscle cells by repressing p21WAF1/Cip1 transcription and cyclin D1-Cdk4-p21WAF1/Cip1 complex formation.

Atherosclerosis and restenosis are common vascular disorders that involve excess proliferation of smooth muscle cells (SMCs) in the artery wall. In this study we demonstrate the anti-mitogenic, pro-apoptotic role of the zinc finger transcription factor Sp1 in vascular SMCs and define the underlying molecular mechanism via its capacity to repress the expression of the cyclin-dependent kinase inhibitor p21WAF1/Cip1 at the level of transcription, mRNA, and protein. SMC proliferation inducible by a dominant-negative mutant form of Sp1 was abrogated by antisense strategies targeting p21WAF1/Cip1. Conversely, antisense p21WAF1/Cip1 induced apoptosis in SMCs overexpressing dominant-negative-Sp1. p21WAF1/Cip1 overexpression alone stimulated proliferation and inhibited apoptosis. Sp1 down-regulated p21WAF1/Cip1 expression in SMCs. Sp1 blocked assembly of cyclin D1-Cdk4-p21WAF1/Cip1 complex formation whose integrity is critical for G1->S transition. Moreover, Rb phosphorylation, which lies immediately downstream of the cyclin D1-Cdk4-p21WAF1/Cip1 complex, was blocked either by Sp1 overexpression or antisense p21WAF1/Cip1. These findings, using complementary approaches, demonstrate the inverse relationship between Sp1 and p21WAF1/Cip1 in SMCs and the capacity of Sp1 to regulate SMC proliferation and apoptosis via its repression of p21WAF1/Cip1.

Animals↗
PubMed2001

TNF-alpha downregulates murine hepatic growth hormone receptor expression by inhibiting Sp1 and Sp3 binding.

Children with chronic inflammatory diseases experience growth failure and wasting. This may be due to growth hormone resistance caused by cytokine-induced suppression of growth hormone receptor (GHR) gene expression. However, the factors governing inflammatory regulation of GHR are not known. We have reported that Sp1 and Sp3 regulate hepatic GHR expression. We hypothesized that TNF-alpha suppresses GHR expression by inhibiting Sp1/Sp3 transactivators. LPS administration significantly reduced murine hepatic GHR expression, as well as Sp1 and Sp3 binding to GHR promoter cis elements. TNF-alpha was integral to this response, as LPS did not affect hepatic Sp1/Sp3 binding or GHR expression in TNF receptor 1-deficient mice. TNF-alpha treatment of BNL CL.2 mouse liver cells reduced Sp1 and Sp3 binding to a GHR promoter cis element and downregulated activity of a GHR promoter-driven luciferase reporter. Combined mutations within adjacent Sp elements eliminated GHR promoter suppression by TNF-alpha without affecting overall nuclear levels of Sp1 or Sp3 proteins. These studies demonstrate that murine GHR transcription is downregulated by LPS, primarily via TNF-alpha-dependent signaling. Evidence suggests that inhibition of Sp transactivator binding is involved. Further investigation of these mechanisms may identify novel strategies for preventing inflammatory suppression of growth.

Animals↗
PubMed1998

Transcription factors Sp1 and Sp3 alter vascular endothelial growth factor receptor expression through a novel recognition sequence.

Kinase domain receptor (KDR) is a high affinity, endothelial cell-specific, autophosphorylating tyrosine kinase receptor for vascular endothelial growth factor. This transcriptionally regulated receptor is a critical mediator of endothelial cell (EC) growth and vascular development. In this study, we identify a DNA element modulating KDR promoter activity and evaluate the nuclear binding proteins accounting for a portion of the cell-type specificity of the region. KDR promoter luciferase activity was retained within -85/+296 and was 10-30-fold higher in EC than non-EC. Electrophoretic mobility shift assays demonstrated specific nuclear protein binding to -85/-64, and single point mutations suggested important binding nucleotides between -79/-68 with five critical bases between -74/-70 (5'-CTCCT-3'). DNA-protein complexes were displaced by Sp1 consensus sequence oligodeoxynucleotides and supershifted by Sp1- and Sp3-specific antibodies. Sp1 and Sp3 protein in EC nuclear extracts bound the -79/-68 region even when all surrounding classic Sp1 recognition sites were removed. Sp1 protein in nuclear extracts was 4-24-fold higher in EC than non-EC, whereas Sp3 was 3-7-fold higher. Sp1/Sp3 ratios in EC were 2-10-fold higher. Overexpression of Sp1 protein increased KDR promoter activity 3-fold in both EC and non-EC, whereas simultaneous co-expression of Sp3 attenuated this response. An Sp1 consensus sequence cis element "decoy" reduced EC KDR promoter activity and mRNA expression by 85 and 69%, respectively. An antisense phosphorothioate oligodeoxynucleotide to Sp1 inhibited Sp1 and KDR protein expression by 66 and 68%, respectively, without changing Sp3 protein expression. These data illustrate that Sp1 and Sp3 modulate KDR promoter activity through a novel recognition binding sequence. However, since Sp1-mediated promoter activation is attenuated by Sp3, endothelial selective KDR promoter activity may be partially regulated by variations in the Sp1/Sp3 ratio.

Base Sequence↗
PubMed2005

Inhibition of Sp1 activity by a decoy PNA-DNA chimera prevents urokinase receptor expression and migration of breast cancer cells.

Sp1 regulates the activation of many genes involved in tumor growth, apoptosis, and angiogenesis. We have previously shown the involvement of Sp1 in the up-regulation of urokinase receptor (uPAR) expression, a key molecule in tumor invasion and metastasis. Here, we investigated whether a marked down-regulation of Sp1 activity may inhibit uPAR expression and migration ability of MDA-MB-231 breast cancer cells. To this end, we tested the decoy ability of a novel peptide nucleic acid (PNA)-DNA chimera which carries a central DNA strand, containing Sp1-binding sequence, covalently linked to two PNA fragments at both ends (PNA-DNA-PNA, PDP). The chimera was synthesized, annealed with complementary DNA (PDP-DNA), and then tested for its ability to bind Sp1 both in vitro and in living MDA-MB-231 breast cancer cells in the presence of urokinase (uPA). This PDP-DNA decoy molecule efficiently competes for the binding to endogenous Sp1 in nuclear extracts, and upon transfection with liposomal vectors, causes a marked decrease of available Sp1 in both untreated and uPA-treated MDA-MB-231 cells. Accordingly, both uPA-dependent enhancement of uPAR expression and cell migration were strongly reduced in transfected cells. Interestingly, a detectable inhibitory effect is also observed in breast cancer cells exposed to PDP-DNA in the absence of transfection reagents. Finally, the inhibitory effect of PDP-DNA appeared to be stronger than that observed with oligonucleotides carrying Sp1 consensus sequence. Our findings show that this novel PNA-DNA chimera, containing Sp1 consensus sequence, effectively inhibits Sp1 activity, uPAR expression, and motility of breast cancer cells indicating its potential therapeutic use to prevent tumor dissemination.

Breast Neoplasms↗
PubMed1989

Suppressive biological activity of a synthetic pentapeptide on highly enriched human and murine marrow hematopoietic progenitors: synergism with recombinant human tumor necrosis factor-alpha and interferon-gamma.

The synthetic pentapeptide pGlu-Glu-Asp-Cys-Lys (SPI) was evaluated in vitro alone and in combination with recombinant human tumor necrosis factor-alpha (TNF-alpha) and/or interferon-gamma (IFN-gamma) for effects on colony formation by hematopoietic progenitor cells (HPC) present in low-density (LD), nonadherent low-density T-lymphocyte-depleted (NALT-), and highly enriched sorted progenitor cells from normal human bone marrow. Progenitor cells in NALT- fractions were further enriched by cell sorting using two-color fluorescence on a Coulter Epics 753 flow cytometry apparatus with My10 and HLA-DR monoclonal antibodies. The sorted My10 DR+ progenitor cell population had a cloning efficiency of up to 38% for granulocyte-macrophage colony-forming units (CFU-GM), erythroid burst-forming units (BFU-E), and multipotential colony-forming units (CFU-GEMM). SP1 inhibited, by up to 86%, each of the colony-forming cells in a dose-dependent fashion. The sensitivities of the different progenitor cells to inhibition by the pentapeptide were the same when My10 DR+ marrow cells were used, and the progenitor cells in the My10 DR+ fraction were more sensitive than the cells in the LD or NALT- fraction to inhibition by SP1. The suppressive activity of SP1 on purified HPC was confirmed when 10(-3) M SP1 completely inhibited colony and cluster formation from a population of mouse bone marrow cells in which one of two cells was a CFU-GM. The effects of SP1 were not absolutely cell-cycle-specific for human HPC, but the non-S-phase cells were less sensitive than the S-phase cells to the suppressive effects of SP1. SP1 synergized with TNF-alpha and/or IFN-gamma to inhibit proliferation of progenitor cells using both LD or My10 DR+ human marrow cells stimulated by recombinant human interleukin 3 (IL-3). These studies suggest that the suppressive effect of SP1 occurs in the absence of certain accessory cells (e.g., monocytes and T-lymphocytes), that this effect may be mediated directly at the level of the HPC, and that this pentapeptide can be considered a candidate modulatory molecule for HPC proliferation.

Animals↗
PubMed2004

Sp1-targeted inhibition of gene transcription by WP631 in transfected lymphocytes.

The binding of Sp1 transcription factor to DNA is considered a potential target for small ligands designed to interfere with gene transcription. We attempted to distinguish the direct inhibition of the Sp1-binding to DNA in vivo (cell culture) from more indirect effects due to the network of pathways that modulate cell cycle progression, which may decrease transcription without direct interference with Sp1-DNA interactions. We tested whether the Sp3 protein, whose putative binding sequence overlaps the Sp1 site, can inhibit Sp1-activated transcription and interfere with drug-DNA interactions. A well-characterized model system consisting of a wtGLUT1 (wild-type glucose transporter 1) gene promoter, or a mutated mut2GLUT1 promoter, linked to a CAT (chloramphenicol acetyltransferase) reporter gene, was used to analyze the effects of overexpressed Sp1 and Sp3 transcription factors in transiently transfected Jurkat T lymphocytes. Bisanthracycline WP631, a potent inhibitor of Sp1-activated transcription in vitro, was assayed for its ability to specifically inhibit transcription in transfected Jurkat T lymphocytes. The mut2GLUT1 promoter was used to further discriminate between the WP631 interference with Sp1-DNA complexes and Sp3-induced inhibition, since the Sp3-binding site is canceled in this promoter and replaced by a high-affinity binding site for WP631.

Cell Division↗
PubMed2004

Thrombin stimulates IL-6 and IL-8 expression in cytomegalovirus-infected human retinal pigment epithelial cells.

Recently, we reported that thrombin specifically stimulates protease-activated receptor-1 (PAR-1) signaling in RPE entailing inhibition of Sp1 dependent HCMV replication. We now studied whether thrombin modulates the expression of the proinflammatory cytokine/chemokines IL-6 and IL-8 in mock- and cytomegalovirus-infected human retinal pigment epithelial cells (RPE). Our data show that thrombin/PAR-1 stimulates IL-6 and IL-8 gene transcription and protein secretion in both mock- and HCMV-infected RPE. Thrombin/PAR-1-mediated signaling stimulated PKC and NF-kappaB-dependent IL-6 and IL-8 gene expression via phosphoinositide 3-kinase and further downstream via p42/44 and p38 MAPKs. Thus, thrombin/PAR-1-mediated IL-6/IL-8 gene expression is uncoupled from Sp1 inhibition and may support proinflammatory pathomechanisms probably involved in hemorrhage/HCMV retinitis progression.

Cytomegalovirus↗
PubMed2006

Sumoylation inhibits cleavage of Sp1 N-terminal negative regulatory domain and inhibits Sp1-dependent transcription.

Sp1 is a ubiquitously expressed transcription factor that binds GC-rich cis elements. Many posttranslational modifications have been implicated in the regulation of Sp1 activity. We now provide evidence for a novel mechanism of Sp1 regulation involving the small ubiquitin-like modifier (SUMO-1). Western blot analysis revealed a high molecular mass Sp1 of 125 kDa that is stabilized by a selective SUMO hydrolase inhibitor and destabilized by a specific SUMO-1 hydrolase. The covalent modification of Sp1 by endogenous SUMO-1 and SUMO-1 that has been fused to green fluorescent protein was demonstrated using transient transfection assays. A high probability sumoylation consensus motif, VK(16)IE(18), is located within the N-terminal negative regulatory domain of Sp1. Either arginine substitution for lysine 16 (Sp1(K16R)) or alanine substitution for glutamic acid 18 (Sp1(E18A)), abrogated Sp1 sumoylation. In vitro SUMO-1 covalently bound affinity-purified GST-Sp1, but not GST-Sp1(K16R). In vivo Sp1 was determined to be N-terminally cleaved, while Sp1(K16R) could not be cleaved indicating that sumoylation and cleavage are coupled through the key regulatory lysine 16. This coupling was evident by the demonstration of an inverse relationship between cellular SUMO-modified Sp1 and N-terminally cleaved Sp1. Compared with Sp1, sumoylation-deficient Sp1(E18A) exhibited enhanced cleavage and was a better transcriptional activator, while constitutively SUMO-1-modified Sp1 was deficient in proteolytic processing and repressed Sp1 transcriptional activity. The repressive effect of sumoylation on Sp1 activity is emphasized through the use of a GAL4 based transactivation assay. A model is proposed defining a mechanism by which sumoylation preserves the integrity of a negative regulatory domain thereby allowing for the inhibition of Sp-dependent transcription.

Amino Acid Sequence↗
PubMed1998

The promyelocytic leukemia protein interacts with Sp1 and inhibits its transactivation of the epidermal growth factor receptor promoter.

The promyelocytic leukemia protein (PML) is a nuclear phosphoprotein with growth- and transformation-suppressing ability. Having previously shown it to be a transcriptional repressor of the epidermal growth factor receptor (EGFR) gene promoter, we have now shown that PML's repression of EGFR transcription is caused by inhibition of EGFR's Sp1-dependent activity. On functional analysis, the repressive effect of PML was mapped to a 150-bp element (the sequences between -150 and -16, relative to the ATG initiation site) of the promoter. Transient transfection assays with Sp1-negative Drosophila melanogaster SL2 cells showed that the transcription of this region was regulated by Sp1 and that the Sp1-dependent activity of the promoter was suppressed by PML in a dose-dependent manner. Coimmunoprecipitation and mammalian two-hybrid assays demonstrated that PML and Sp1 were associated in vivo. In vitro binding by means of the glutathione S-transferase (GST) pull-down assay, using the full-length and truncated GST-Sp1 proteins and in vitro-translated PML, showed that PML and Sp1 directly interacted and that the C-terminal (DNA-binding) region of Sp1 and the coiled-coil (dimerization) domain of PML were essential for this interaction. Analysis of the effects of PML on Sp1 DNA binding by electrophoretic mobility shift assay (EMSA) showed that PML could specifically disrupt the binding of Sp1 to DNA. Furthermore, cotransfection of PML specifically repressed Sp1, but not the E2F1-mediated activity of the dihydrofolate reductase promoter. Together, these data suggest that the association of PML and Sp1 represents a novel mechanism for negative regulation of EGFR and other Sp1 target promoters.

Binding Sites↗
PubMed1994

The retinoblastoma gene product RB stimulates Sp1-mediated transcription by liberating Sp1 from a negative regulator.

Studies have demonstrated that the retinoblastoma susceptibility gene product, RB, can either positively or negatively regulate expression of several genes through cis-acting elements in a cell-type-dependent manner. The nucleotide sequence of the retinoblastoma control element (RCE) motif, GCCACC or CCACCC, and the Sp1 consensus binding sequence, CCGCCC, can confer equal responsiveness to RB. Here, we report that RB activates transcription of the c-jun gene through the Sp1-binding site within the c-jun promoter. Preincubation of crude nuclear extracts with monoclonal antibodies to RB results in reduction of Sp1 complexes in a mobility shift assay, while addition of recombinant RB in mobility shift assay mixtures with CCL64 cell extracts leads to an enhancement of DNA-binding activity of SP1. These results suggest that RB is directly or indirectly involved in Sp1-DNA binding activity. A mechanism by which RB regulates transactivation is indicated by our detection of a heat-labile and protease-sensitive Sp1 negative regulator(s) (Sp1-I) that specifically inhibits Sp1 binding to a c-jun Sp1 site. This inhibition is reversed by addition of recombinant RB proteins, suggesting that RB stimulates Sp1-mediated transactivation by liberating Sp1 from Sp1-I. Additional evidence for Sp1-I involvement in Sp1-mediated transactivation was demonstrated by cotransfection of RB, GAL4-Sp1, and a GAL4-responsive template into CV-1 cells. Finally, we have identified Sp1-I, a approximately 20-kDa protein(s) that inhibits the Sp1 complexes from binding to DNA and that is also an RB-associated protein. These findings provide evidence for a functional link between two distinct classes of oncoproteins, RB and c-Jun, that are involved in the control of cell growth, and also define a novel mechanism for the regulation of c-jun expression.

3T3 Cells↗
PubMed2004

The repressor element silencing transcription factor (REST)-mediated transcriptional repression requires the inhibition of Sp1.

The terminal differentiation of neuronal and pancreatic beta-cells requires the specific expression of genes that are targets of an important transcriptional repressor named RE-1 silencing transcription factor (REST). The molecular mechanism by which these REST target genes are expressed only in neuronal and beta-cells and are repressed by REST in other tissues is a central issue in differentiation program of neuronal and beta-cells. Herein, we showed that the transcriptional factor Sp1 was required for expression of most REST target genes both in insulin-secreting cells and neuronal-like cells where REST is absent. Inhibition of REST in a non-beta and a non-neuronal cell model restored the transcriptional activity of Sp1. This activity was also restored by trichostatin A indicating the requirement of histone deacetylases for the REST-mediated silencing of Sp1. Conversely, exogenous introduction of REST blocked Sp1-mediated transcriptional activity. The REST inhibitory effect was mediated through its C-terminal repressor domain, which could interact with Sp1. Taken together, these data show that the inhibition of Sp1 by REST is required for the silencing of its target genes expression in non-neuronal and in non-beta-cells. We conclude that the interplay between REST and Sp1 determines the cell-specific expression of REST target genes.

Animals↗
PubMed2006

Irisolidone, an isoflavone metabolite, represses JC virus gene expression via inhibition of Sp1 binding in human glial cells.

Progressive multifocal leukoencephalopathy (PML) is a fatal demyelinating disease that results from an oligodendrocyte infection caused by the JC virus. Therefore, inhibiting the expression of JC virus is important for preventing and/or treating PML. This study found that irisolidone, an isoflavone metabolite, significantly inhibited the JC virus expression in primary cultured human astrocytes and glial cell lines. Studies examining the underlying mechanism revealed that a mutation of the Sp1 binding site downstream of the TATA box (Sp1-II) dramatically diminished the inhibitory activity of irisolidone. In addition, an irisolidone treatment repressed Sp1 binding to Sp1-II site, which is important for the basal JC virus promoter activity. The results suggest that the inhibitory effect of irisolidone against the JC virus may be attributed at least in part to the suppression of Sp1 binding to the JC virus promoter region. Therefore, the inhibition of the JC virus expression by irisolidone might provide therapeutic potential for PML caused by the JC virus.

Base Sequence↗
PubMed2006

The dioxin receptor is silenced by promoter hypermethylation in human acute lymphoblastic leukemia through inhibition of Sp1 binding.

The transcription factor aryl hydrocarbon receptor (AhR) has relevant functions in cell proliferation. Interestingly, the AhR can either promote or inhibit proliferation depending on the cell phenotype. Although recent data reveal potential pathways for AhR signaling in cell proliferation, the mechanisms that regulate its activity in tumor cells remain unknown. Here, we have analyzed promoter hypermethylation as a potential mechanism controlling AhR expression in human tumor cells. AhR promoter CpG methylation was sporadic in a panel of 19 tumor cell lines except for the chronic myeloid leukemia (CML) K562 and the acute lymphoblastic leukemia (ALL) REH. When compared with normal lymphocytes, REH had very low constitutive AhR expression that could be attributed to promoter hypermethylation since treatment with the DNA demethylating agent 5-aza-2'-deoxycitidine (AZA) significantly increased AhR mRNA and protein. These results in leukemia-derived cell lines were further confirmed in primary ALL, where 33% of the patients (7/21) had AhR promoter hypermethylation. Chromatin immunoprecipitation (ChIP) showed that methylation impaired binding of the transcription factor Sp1 to the AhR promoter, thus providing a mechanism for AhR downregulation in REH cells. Therefore, promoter hypermethylation represents a novel epigenetic mechanism downregulating AhR activity in hematological malignancies such as ALL.

Azacitidine↗
PubMed2005

Artificial zinc finger fusions targeting Sp1-binding sites and the trans-activator-responsive element potently repress transcription and replication of HIV-1.

Tat activates transcription by interacting with Sp1, NF-kappaB, positive transcription elongation factor b, and trans-activator-responsive element (TAR). Tat and Sp1 play major roles in transcription by protein-protein interactions at human immunodeficiency virus, type 1 (HIV-1) long terminal repeat. Sp1 activates transcription by interacting with cyclin T1 in the absence of Tat. To disrupt the transcription activation by Tat and Sp1, we fused Sp1-inhibiting polypeptides, zinc finger polypeptide, and the TAR-binding mutant Tat (TatdMt) together. A designed or natural zinc finger and Tat mutant fusion was used to target the fusion to the key regulatory sites (GC box and TAR) on the long terminal repeat and nascent short transcripts to disrupt the molecular interaction that normally result in robust transcription. The designed zinc finger and TatdMt fusions were targeted to the TAR, and they potently repressed both transcription and replication of HIV-1. The Sp1-inhibiting POZ domain, TatdMt, and zinc fingers are key functional domains important in repression of transcription and replication. The designed artificial zinc fingers were targeted to the high affinity Sp1-binding site, and by being fused with TatdMt and POZ domain, they strongly block both Sp1-cyclin T1-dependent transcription and Tat-dependent transcription, even in the presence of excess expressed Tat.

Binding Sites↗
PubMed1994

Effect of abasic linker substitution on triplex formation, Sp1 binding, and specificity in an oligonucleotide targeted to the human Ha-ras promoter.

A region of the human Ha-ras promoter (-8 to -28) which contains two of the three Sp1 binding sites essential for transcriptional activity forms a sequence specific oligonucleotide-directed pur*pur:pyr triple helix. The relative binding of oligonucleotides containing different substitutions, including an abasic propanediol linker, over three potentially destabilizing C:G interruptions in the otherwise poly G:poly C target was examined. DNase I footprint titrations reveal that substitution of the positively charged abasic propanediol linker results in approximately ten fold greater binding than cytosine substitution which in turn provides greater sequence specific binding than substitution of a guanine in the third strand oligonucleotide over the C:G interruptions. Protein binding assays demonstrate that triplex formation by the linker substituted oligomer (HR21Xap) is less effective in inhibiting Sp1 binding than the cytosine substituted oligomer (HR21ap) both to the target sequence as well as an upstream sequence. As an indication of the effect of linker substitution and targeting consensus Sp1 sites on triplex specificity, the relative ability of the Ha-ras promoter targeted oligonucleotides to interact with non-target Sp1 sequences within the Ha-ras promoter as well as in the DHFR promoter and HIV-1 LTR was also investigated. At concentrations which afford complete DNase I protection of the target sequence, HR21ap does not bind to the non-target sequences while HR21Xap interacts weakly only at a distal site in the DHFR promoter. Also, HR21ap as well as HR21Xap are specific in their inhibition of Sp1 binding. These results suggest that the propanediol linker is able to skip over interruptions in a target sequence thereby stabilizing triplex but, slightly compromises sequence specificity and the ability to inhibit Sp1 binding to the Ha-ras promoter.

Base Composition↗
PubMed2006

Sp3 inhibits Sp1-mediated activation of the cardiac troponin T promoter and is downregulated during pathological cardiac hypertrophy in vivo.

Combinatorial interactions between cis elements and trans-acting factors are required for regulation of cardiac gene expression during normal cardiac development and pathological cardiac hypertrophy. Sp factors bind GC boxes and are implicated in recruitment and assembly of the basal transcriptional complex. In this study, we show that the cardiac troponin T (cTnT) promoter contains a GC box that is necessary for basal and cAMP-mediated activity of cTnT promoter constructs transfected in embryonic cardiomyocytes. Cardiac nuclear proteins bind the cTnT GC box in a sequence-specific fashion and consist of Sp1, Sp2, and Sp3 protein factors. By chromatin immunoprecipitation, Sp1 binds the cTnT promoter "in vivo." Cotransfected Sp1 trans-activates the cTnT promoter in cardiomyocytes in culture. Sp3 represses Sp1-mediated transcriptional activation of the cTnT gene in embryonic cardiomyocytes. Sp3 repression of Sp1-mediated cTnT promoter activation is dose dependent, inferring a mechanism of competitive binding/inhibition. To evaluate the role of Sp factors in cardiac gene expression in vivo, we have established a clinically relevant animal model of pathological cardiac hypertrophy where the fetal cardiac program is activated. In this animal model, cardiac hypertrophy results from increased left-right shunting, volume loading of the left ventricle, and pressure loading of the right ventricle. Sp1 expression is increased in all four hypertrophied cardiac chambers, whereas Sp3 expression is diminished. This observation is consistent with the in vitro activating function of Sp1 and inhibitory effects of Sp3 on activity of cTnT promoter constructs. Sp factor levels are modulated during the hypertrophic cardiac program in vivo.

Animals↗