Cloning of GT box-binding proteins: a novel Sp1 multigene family regulating T-cell receptor gene expression

Developmental regulation of V(D)J recombination at the TCR alpha/delta locus

The T-cell receptor (TCR) alpha/delta locus includes a large number of V, D, J and C gene segments that are used to produce functional TCR delta and TCR alpha chains expressed by distinct subsets of T lymphocytes. V(D)J recombination events within the locus are regulated as a function of developmental stage and cell lineage during T-lymphocyte differentiation in the thymus. The process of V(D)J recombination is regulated by cis-acting elements that modulate the accessibility of chromosomal substrates to the recombinase. Here we evaluate how the assembly of transcription factor complexes onto enhancers, promoters and other regulatory elements within the TCR alpha/delta locus imparts developmental control to VDJ delta and VJ alpha rearrangement events. Furthermore, we develop the notion that within a complex locus such as the TCR alpha/delta locus, highly localized and region-specific control is likely to require an interplay between positive regulatory elements and blocking or boundary elements that restrict the influence of the positive elements to defined regions of the locus.

Isolation and characterization of the human prosaposin promoter

Prosaposin is a multifunctional protein that encodes four glycoproteins, named saposins A, B, C and D. They participate in the catabolism of glycosphingolipids in lysosomes. When secreted, intact prosaposin may function as a neuritogenic factor. Human and mouse prosaposin displayed similar temporal and spatial regulation of expression. To gain insight into the transcriptional regulation of this locus, the 5' region was characterized from the human prosaposin gene. The putative human promoter was shown to be TATA-less, i.e. it belonged to the TATA-less housekeeping gene family. The transcription initiation sites were localized to -23, -27, -31 and -83bp 5' to ATG, compared to -87 and -94bp in the mouse. In SK-N-SH neuroblastoma cells, positive regulatory elements were detected -343 to -813bp upstream of ATG. A negative regulatory region existed between -813 and -2500bp using SK-N-SH, H441 and NS20Y cells. EMSA and DNA-footprint analysis showed that Sp1 and Sp3 are involved in human prosaposin gene regulation. Compared to the mouse promoter, the human promoter is missing a Sp1 cluster within a 310-bp upstream segment, and has AP-1, Oct-1 and two RORalpha sites that are protected from DNaseI by selected nuclear extracts.

Regulation of mouse Ah receptor (Ahr) gene basal expression by members of the Sp family of transcription factors

The aromatic hydrocarbon receptor (AHR) is a ligand-activated transcription factor that regulates the expression of several drug-metabolizing enzymes and has been implicated in immunosuppression, teratogenesis, cell-specific hyperplasia, and certain types of malignancies and toxicities. The mouse Ahr gene 5' proximal promoter region, which contains four potential Sp1 motifs, is required for efficient basal expression. Using a fragment spanning the region from nt -174 to +70 of the Ahr promoter, we found that four regions corresponding to four Sp1 sites were protected from DNase I digestion using nuclear extracts from MLE-12 (lung), F9 (embryonal carcinoma), Hepa-1 (hepatoma), and 41-5a (epidermal) cells. The Hepa-1 and F9 cell lines were shown by reverse transcriptase-polymerase chain reaction and Western blot to contain mRNA and protein for Sp1 and Sp3, but not Sp2 and Sp4. In electrophoretic mobility shift assays using oligonucleotide probes corresponding to the four Ahr Sp1 sites, nuclear extracts from Hepa-1 and F9 cells formed complexes that were determined immunologically to contain both Sp1 and Sp3 protein. The two Ahr proximal Sp1 sites (A and B) were shown to bind both Sp1 and Sp3 proteins, whereas the more distal sites (C and D) bound only Sp1. Competition gel shift experiments showed that sites A and B had 10-fold higher affinity for Sp factors than did sites C and D. To determine the transactivation potential of each of the four Ahr Sp1 sites, we fused the Ahr promoter to a luciferase (LUC) reporter gene and transfected the construct into the Drosophila cell line Schneider-2, which contains no Sp1 or Sp1-like factors. Cotransfection of this construct with expression plasmids for each of the Sp factors revealed that Sp3 was approximately 1.6-fold more efficient than Sp1 in Ahr transactivation. Mutation of the four Sp1 sites individually and in combination demonstrated that each site contributes to the overall level of expression of the reporter gene and that interactions between these sites play a minor role in regulation of the Ahr-LUC construct. These results suggest that basal Ahr expression may be regulated by the expression and distribution of Sp1-like factors.

Multiple promoter elements interact to control the transcription of the potassium channel gene, KCNJ2

Potassium channels play important roles in shaping the electrical properties of excitable cells. Toward understanding the transcriptional regulation of a member of the inwardly rectifying potassium channel family, we have characterized the genomic structure and 5'-proximal promoter of the murine Kcnj2 gene (also referred to as IRK1 and Kir2.1). The Kcnj2 transcription unit is composed of two exons separated by a 5.5-kilobase pair intron. Deletion analysis of 5'-flanking sequences identified a promiscuously active 172-base pair minimal promoter, whereas expression from a construct containing additional upstream sequences was cell type-restricted. The minimal promoter contained an E box, a Y box, and three GC box consensus elements but lacked both TATA and CCAAT box elements. The activity of the minimal promoter was found to be controlled by a combination of the activities of the transcription factors Sp1, Sp3, and NF-Y. The interplay between Sp1, Sp3, and NF-Y within the architecture of the Kcnj2 promoter, the ubiquitous nature of these trans-acting factors, and the action of tissue-selective repressor element(s) may combine to enable a wide variety of cell types to differentially regulate Kcnj2 expression through transcriptional control.

Modulation of human alpha1(I) procollagen gene activity by interaction with Sp1 and Sp3 transcription factors in vitro

In previous work, we delineated a proximal region of the human alpha1(I) collagen gene (COL1A1) promoter necessary to direct its basal transcription in fibroblasts. This region has potential recognition sites for a variety of DNA-binding proteins. Here, we show that the -129/-107-bp sequence in this region of the promoter, which harbors an inverted CCAAT motif closely linked to a GC-rich direct repeat and is perfectly conserved between mouse and human, specifically bound the transcription factors Sp1, Sp3, and CTF/NF-1 in nuclear extracts from human skin and lung fibroblasts. Drosophila Schneider L2 cells lacking endogenous Sp activity were used to investigate the effect of expression of Sp or CTF/NF-1 transcription factors on COLlAl promoter activity. Expression of Sp1 caused potent trans-activation of a COL1A1 promoter (-174 to +42bp). In contrast, expression of Sp3, which binds to the same recognition sites as Sp1, and CTF/NF-1 stimulated COL1A1 promoter activity only at higher concentrations, and Sp2 did not transactivate. Expression of a 10-fold excess of Sp3, but not CTF/NF-1 or Sp2, abrogated the stimulation of COL1A1 promoter activity induced by Sp1. TGF-beta at concentrations previously shown to increase COL1A1 transcription caused a decrease in the relative amount of Sp3 in fibroblast nuclear extracts. These results suggest that both Sp1 and Sp3 bind to the proximal COL1A1 promoter and stimulate its activity; however, their interaction with each other may result in repression of Sp1-induced COL1A1 transcription. Alterations in the relative amounts or DNA-binding activities of these transcription factors in a cell- or signal-specific manner may contribute to the control of transcription from the COL1A1 promoter. The present findings, and recent observations implicating Sp1 and its homologs in regulating the expression of several collagen genes, suggest that the family of Sp1 transcription factors play a role in physiological and pathological modulation of connective tissue accumulation.

Effect of chronic antidepressant treatment on transcription factor binding activity in rat hippocampus and frontal cortex

1. The effect of chronic antidepressant administration on CRE-, SP1- and GRE-binding activity was studied in rat hippocampus and frontal cortex. 2. Fluoxetine and desipramine (3 and 10 mg/kg/day respectively) were given to rats for 21 consecutive days. The animals were killed 3 hr after the last injection and nuclear extracts were prepared to perform the DNA-protein reaction with consensus CRE, SP1 and GRE oligonucleotides. 3. Gel-shift assays showed that CRE-binding activity was increased in both frontal cortex and hippocampus by chronic fluoxetine treatment. Desipramine, however, only enhanced this activity in the frontal cortex. 4. Chronic fluoxetine decreased SP1-binding activity in the two selected brain regions. Again, desipramine only produced a significant reduction in the frontal cortex. 5. GRE-binding in the hippocampus was only enhanced by desipramine. Since chronic desipramine, and not fluoxetine, is able to increase hippocampal glucocorticoid receptor (GR) expression, interactions of GR with CREB and SP1 may determine the lack of effect of desipramine on binding activity of the two latter transcription factors in this brain region. 6. Overall, the results show a differential and region-specific effect of chronic, and not acute, antidepressant treatment on the DNA-binding activities studied and are consonant with the possible role of changes in gene expression in the mechanism of antidepressant action.

Characterization of the human transcobalamin II promoter. A proximal GC/GT box is a dominant negative element

Deletion and mutagenesis of the 5'-flanking region of the human transcobalamin II (TC II) transfected in human intestinal epithelial Caco-2 cells have revealed that TC II promoter activity is: (a) very weak; (b) restricted to a core region (-29 to -163) that contained multiple transcription initiation sites; (c) not dependent on other potential elements, such as a distally localized CCAAT box, a CF1, a HIP1 binding motif and a MED-1 element; (d) modulated weakly by a positive-acting GC box (-568-GAGGCGGTGC) and strongly by a proximal GC/GT overlapping box (-179 CCCCCGCCCCACCCC). Gel shift and immunosupershift analyses demonstrated that both the positive-acting GC box and the negative-acting GC/GT box were recognized by Sp1 and Sp3. Co-transfection studies using Sp1 and/or Sp3 expression plasmids revealed that while Sp1 stimulated, Sp3 repressed Sp1-mediated transactivation of TC II transcription. The proximal GC/GT box also acted as a negative element in human chronic myelogenous leukemia K-562 and HeLa cells. These results suggest that tissue/cell specific expression of the TC II gene may be controlled by the relative ratios of Sp1 and Sp3 that bind to the GC/GT box and the weak promoter activity of TC II is due to the transcriptional repression caused by the binding of Sp3 to the proximal GC/GT box.

Characterization of the human Megalin/LRP-2 promoter in vitro and in primary parathyroid cells

The gp330/Megalin/LRP-2 protein belongs to the low-density lipoprotein receptor gene family and is believed to function as an endocytic receptor for the uptake of lipoproteins and many other ligands. Other functions of this protein may include a role in calcium sensing in the parathyroid glands and other tissues. In order to study the transcriptional regulation of the human LRP-2 gene, a clone containing the 5'-flanking region was isolated from a genomic DNA library, and a transient transfection protocol for primary bovine parathyroid cells was established. RNA mapping techniques located the transcriptional start site 136 bp upstream of the initiation codon. Transient expression in several cell types, including primary parathyroid cells, and in vitro transcription in HeLa cell nuclear extracts showed that sequences between -120 and -35 were important for activated transcription. This region contains consensus binding sites (GC boxes) for transcription factor Sp1. Mutation of the GC boxes abolished binding of Sp1 in vitro and resulted in reduced transcription in vitro and in transfected cells. Furthermore, Sp1 stimulated transcription when tethered to the LRP-2 core promoter through a heterologous DNA-binding domain. Through site-directed mutagenesis, we identified a novel atypical TATA element with the sequence TAGAAAA. Intriguingly, this sequence motif was shown previously not to mediate transcription in a systematic mutational analysis of the TATA motif. Possible roles of this novel TATA element in the regulation of transcription initiation are discussed. The isolation and characterization of the LRP-2 promoter and the 5'-flanking region and the establishment of a transient expression assay in primary parathyroid cells will facilitate studies on the regulatory mechanisms of the LRP-2 gene and of other genes expressed in the parathyroid glands.

Role of Sp proteins and RORalpha in transcription regulation of murine prosaposin

Prosaposin is the precursor of four low molecular weight sphingolipid-activating proteins (SAPs) or saposins. These four proteins function as intracellular activators of several lysosomal enzymes involved in the degradation of glycosphingolipids, and prosaposin itself has neurite outgrowth effects. Expression of prosaposin is regulated in a temporal and spatial manner with expression in specific brain neurons and visceral cell types. Here a major regulatory fragment was characterized within 310 bp 5' to the transcription start site. Using electrophoretic mobility shift assay (EMSA) and DNA footprinting, members of the Sp family (Sp1, Sp3, and Sp4), the orphan nuclear receptor (RORalpha), and an unknown transcription factor (U; TGGGGGAG) were shown to bind to this region. To evaluate the role of such transcription factor binding sites for this locus, a series of mutant constructs was generated within this region, and their function was evaluated in cultured NS20Y neuroblastoma cells. A 3' Sp1 site, a 5' Sp1/U cluster and the RORalpha binding sites were functional. The data are consistent with a model in which the factors that bind to the Sp1/U cluster and RORE site interact negatively to diminish promoter activity to a background level that is determined primarily by the 3' Sp1 site. These interactions depend on the tissue-specific repertoire of transcription factors leading to differential expression of this locus.

Sp1 cooperates with the ets transcription factor, GABP, to activate the CD18 (beta2 leukocyte integrin) promoter

CD18, the beta chain of the leukocyte integrins, plays a crucial role in immune and inflammatory responses. CD18 is expressed exclusively by leukocytes, and it is transcriptionally regulated during the differentiation of myeloid cells. The ets factors, PU.1 and GABP, bind to three ets sites in the CD18 promoter, which are essential for high level myeloid expression of CD18. We now identify two binding sites for the transcription factor, Sp1, that flank these ets sites. Sp1 is the only factor from myeloid cells that binds to these sites in a sequence-specific manner. Mutagenesis of these sites abrogates Sp1 binding and significantly reduces the activity of the transfected CD18 promoter in myeloid cells. Transfection of Sp1 into Drosophila Schneider cells, which otherwise lack Sp1, activates the CD18 promoter dramatically. GABP also activates the CD18 promoter in Schneider cells. Co-transfection of Sp1 and GABP activates CD18 more than the sum of their individual effects, indicating that these factors cooperate to transcriptionally activate myeloid expression of CD18. These studies support a model of high level, lineage-restricted gene expression mediated by cooperative interactions between widely expressed transcription factors.

Nonmyogenic factors bind nicotinic acetylcholine receptor promoter elements required for response to denervation

Nicotinic acetylcholine receptors (AChRs) belong to a class of muscle proteins whose expression is regulated by muscle electrical activity. In innervated muscle fiber, AChR genes are transcriptionally repressed outside of the synapse, while after denervation they become reexpressed throughout the fiber. The myogenic determination factors (MDFs) of the MyoD family have been shown to play a central role in this innervation-dependent regulation. In the chicken AChR alpha-subunit gene promoter, two E-boxes that bind MDFs are necessary to achieve the enhancement of transcription following muscle denervation. However, the deletion of promoter sequences located upstream to these E-boxes greatly impairs the response to denervation (Bessereau, J. L., Stratford- Perricaudet, L. D., Piette, J., Le Poupon, C. and Changeux, J. P. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 1304-1308). Here we identified two additional cis-regulatory elements of the alpha-subunit gene promoter that cooperate with the E-boxes in the denervation response. One region binds the Sp1 and Sp3 zinc finger transcription factors. The second region binds at least three distinct factors, among which we identified an upstream stimulatory factor, a b-ZIP-HLH transcription factor. We propose that among MDF-responsive muscle promoters, a specific combination between myogenic and nonmyogenic factors specify innervation-dependent versus innervation-independent promoters.

Characterization of two Sp1 binding sites of the human sex determining SRY promoter

To investigate the molecular basis of the human SRY gene regulation, we have examined the significance of two potential binding sites for the transcription factor Sp1 (Sp1A: -124 to -131 and Sp1B: -147 to -154) by DNase I footprinting and gel mobility shift assays. Cotransfection experiments in Drosophila SL2 cells implicated Sp1 protein in the transcriptional activation of the SRY promoter.

Truncation of Sp1 transcription factor by myeloblastin in undifferentiated HL60 cells

When HL60 cells are exposed to 1,25-dihydroxyvitamin D3 (1,25D3), they undergo changes approximating the phenotype of the monocyte. Little is known, however, about the regulation and the mechanisms of this transition. It was previously noted that DNA binding by the Sp1 transcription factor in nuclear extracts of HL60 cells is profoundly altered when these cells are induced to differentiate by 1,25D3. In the present study, we show that in untreated HL60 cells only a truncated, approximately 30-kDa Sp1 fragment, encompassing the C-terminal region, binds to the GC element-containing DNA. Full-length 105-kDa Sp1 protein cannot be detected in these cells, although reverse transriptase-polymerase chain reaction reveals the presence of both 5' and 3' ends of Sp1 mRNA. Following treatment with 10(7) M 1,25D3 for 96 hr or in cells made resistant to 1,25D3 or to 1-beta-D-arabinocytosine, the Sp1 protein can be demonstrated. After an exposure to purified myeloblastin, a serine protease, purified recombinant Sp1 protein and extracts of 1,25D3-treated cells show a pattern of DNA binding similar to the pattern seen using extracts of untreated HL60 cells, indicating that the Sp1 protein is a target for myeloblastin. Because myeloblastin is present in naive HL60 cells and is downregulated during their differentiation, inhibition of proteolysis of these transcription factors seems to provide a mechanism through which differentiating HL60 cells can acquire a new repertoire of gene expression, perhaps for the maintenance of the differentiated phenotype.

Sp family transcription factors regulate expression of rat D2 dopamine receptor gene

The rat D2 dopamine receptor gene is transcribed from a TATA-less promoter that has an initiator-like sequence and several putative Sp1 binding sites. We previously reported that a negative modulator is located between nucleotides -116 and -76 (D2Neg-B) in this gene and that Sp1 as well as another unknown factor bind to this region (Minowa et al., J. Biol. Chem. 269, 11656, 1994). In the present investigation employing the in situ filter detection method, we identified this factor as Sp3. Anti-Sp3 antiserum used in gel-shift assays also revealed that Sp3 binds to the D2Neg-B sequence. Cotransfection of Drosophila Schneider's SL2 cells with Sp3 or Sp1 expression plasmids in the presence of CAT reporter plasmids containing D2 promoter regions demonstrated that Sp1 increased CAT activity in a dose-dependent manner, whereas Sp3, either alone or in the presence of Sp1, failed to activate or repress transcription. On the other hand, using the TATA-containing reporter plasmid BCAT-2, Sp3 coexpression significantly repressed Sp1-induced trans-activation, although Sp3 alone was ineffective. Thus, the transcriptional activity of Sp3 is dependent on the promoter context, and the negative regulation of D2 gene expression appears quite complex and may not depend simply on known DNA-protein interactions involving only Sp1 and Sp3.

Transcriptional regulation of the human nonmuscle myosin II heavy chain-A gene. Identification of three clustered cis-elements in intron-1 which modulate transcription in a cell type- and differentiation state-dependent manner

In an attempt to identify cis-acting elements for transcriptional regulation of the human nonmuscle myosin II heavy chain (MHC)-A gene, the region extending 20 kilobases (kb) upstream and 40 kb downstream from the transcription start sites, which includes the entire 37-kb intron 1, was examined. Using transient transfection analysis of luciferase reporter constructs, a 100-base pair (bp) region (N2d) in intron 1, located 23 kb downstream from the transcriptional start sites, has been found to activate transcription in a cell type- and differentiation state-dependent manner. Maximum activity (approximately 20-fold) is seen in NIH 3T3 fibroblasts and intermediate activity (7-fold) in proliferating and undifferentiated C2C12 myoblasts. In contrast, this region is almost inactive in terminally differentiated C2C12 myotubes, in which endogenous nonmuscle MHC-A expression is down-regulated. Gel mobility shift assays and methylation interference analyses were performed using NIH 3T3 nuclear extracts to determine the protein-binding elements for transcription factors. Three binding elements have been identified within the N2d region. Antibody-supershift experiments, as well as competition experiments using consensus binding sequences for specific transcription factors, revealed that the most 5'-element, C (GGGAGGGGCC) is recognized specifically and exclusively by Sp1 and Sp3 transcriptional factors. Element C is immediately followed by a novel element, A (GTGACCC). A third element, F (GTGTCAGGTG), which contains an E-box, is located 50 bp 3' to element A. Element F can be recognized partially by upstream stimulatory factors, USF1 and/or USF2. Transfection studies with luciferase reporter constructs which include mutations in all three elements in various combinations demonstrate that the A and C binding factors cooperatively activate transcriptional activity in NIH 3T3 cells. The F binding factor shows an additive effect on transcription.

Requirement of a GT box (Sp1 site) and two Ets binding sites for vascular endothelial cadherin gene transcription

Vascular endothelial cadherin (VE cadherin) gene encodes a Ca2+-dependent cell adhesion molecule required for the organization of interendothelial junctions. This gene is exclusively and constitutively expressed in endothelial cells. Previous data with transgenic mice revealed that the transcriptional regulatory elements present within a -2486/+24 DNA fragment of mouse VE cadherin gene mimic the tissue-specific activity of the endogenous promoter. In this study, we analyzed elements implicated in the function of the proximal regulatory region. Electrophoretic mobility shift assay identified a GT-rich sequence (positions -49/-39) interacting with factors related to the Sp1 family. Point mutations abolished the binding of nuclear proteins in vitro and drastically diminished the activity of the promoter in transient transfection assay. Supershift assays with antibodies against proteins of the Sp1 family revealed that Sp1 and Sp3 interact with this region of the VE cadherin promoter. Furthermore, two GGAA motifs, located at positions -93/-90 and -109/-106, were shown to interact with nuclear factors. Site-directed mutagenesis of these sequences demonstrated that these Ets binding sites are essential for promoter activity. In vitro binding assays in the presence of various antisera suggest that Erg is one of the proteins interacting with the -109/-106 site.

An Sp1-binding silencer element is a critical negative regulator of the megakaryocyte-specific alphaIIb gene

The Sp1 family of transcription factors are often involved in the regulated expression of TATA-less genes, frequently enhancing gene transcription. In this paper, we demonstrate that an Sp1-binding element inhibits the expression of the megakaryocyte-specific alphaIIb gene in all cell lines tested and that this inhibition is actively overcome only in megakaryocyte-like cell lines. We had noted previously in primary megakaryocytes that a 50-base pair (bp) deletion from -150 to -101 bp in the rat alphaIIb promoter region resulted in increased expression. We now show that deletion of this region markedly increased expression in both megakaryocytic and non-megakaryocytic cell lines, eliminating the tissue specificity of the alphaIIb promoter. Electrophoretic mobility shift assays (EMSA) defined a single complex, which bound to a -145 to -125 bp subregion. Point mutations within this region, localized the critical point of binding around bases -136/-135, and expression studies showed that introduction of the -136/-135 mutation into the rat alphaIIb promoter had a comparable result to that seen with the 50-bp deletion. EMSA studies with the homologous human alphaIIb promoter region gave an identical migrating band. Southwestern blots of HeLa nuclear proteins with both the rat -145 to -125 DNA and its human homologue bound to a single approximately 110-kDa protein, the known molecular weight of Sp1. Confirmation that this region of the alphaIIb gene promoter bound Sp1 was accomplished using EMSA studies with an Sp1 consensus probe, anti-Sp1 and -Sp3 antibodies, and recombinant Sp1 protein. Further support for the role of Sp1 in the silencing of the alphaIIb promoter was obtained using a Gal4 binding site substitution for the silencer region of alphaIIb and co-expression of near full-length Sp1/Gal4 fusion protein expression vectors. Ectopic reinsertion of the -150 to -101 bp region, back into the -150 to -101 bp deleted promoter, enhanced rather than decreased expression, suggesting that Sp1's inhibitory role at -136/-135 depends on its local interactions. In summary, we believe that we have identified a cross-species, non-consensus Sp1-binding site that binds Sp1 and that acts as a silencer of alphaIIb expression in many cell lines. A model is presented as to how this Sp1-binding silencer element contributes to the megakaryocyte-specific expression of alphaIIb gene.

SP1/SP3-binding sites and adjacent elements contribute to basal and cyclic adenosine 3',5'-monophosphate-stimulated transcriptional activation of the rhesus growth hormone-variant gene in trophoblasts

Transcriptional activation of the rhesus monkey GH-variant gene in syncytiotrophoblasts is developmentally regulated by trophoblast-specific and cAMP-responsive mechanisms. Progressive deletions of 5'-flanking DNA defined the most proximal 140 bp as the minimal region retaining full cAMP-stimulated mGH-V transcription. To identify the regions of this promoter critical for transcription, transient transfections of reporter plasmids containing systematic 10 base mutations throughout this proximal region were performed. Mutation of the region from -140/-131 decreased transcription in syncytiotrophoblasts by 50%, and gel mobility-shift analyses demonstrated that Sp1 and Sp3 bound to a region containing a GGGAGG motif at -136/-131. Mutation of the -62/-53 region decreased transcriptional activation by 66-99%, and Sp1 and Sp3 bound to a GGTGGG motif overlapping this region (at -65/-60). Selective mutation of this Sp1/Sp3 site decreased basal transcription by approximately 80%, and cAMP-stimulated transcription by up to 75% (with the greatest effect in primary syncytiotrophoblast cultures), indicating that the Sp1/Sp3 site is critical for transcriptional activation. Mutations in the regions adjacent to the Sp1/Sp3 sites (-130/-111 and -52/-43) also dramatically reduced (by 75%) transcriptional activation in trophoblasts. We conclude that two Sp1/Sp3 sites as well as additional elements directly adjacent to these sites contribute to trophoblast-specific cAMP-responsiveness of the mGH-V proximal promoter.

Activation of the ferritin H enhancer, FER-1, by the cooperative action of members of the AP1 and Sp1 transcription factor families

We have previously reported that the adenovirus E1A oncogene represses the transcription of the H subunit of the mouse ferritin gene. Subsequent analyses defined FER-1, a 37-nucleotide sequence located 4.1 kilobases proximal to the start site of transcription, as the target of E1A-mediated transcriptional repression and as an enhancer of the ferritin H gene. FER-1 is composed of an AP1-like sequence followed by an element with dyad symmetry. To achieve maximal enhancer activity and transcriptional repression by E1A, both elements were essential. Using gel retardation assays, we now demonstrate that the binding complex for the AP1-like sequence of FER-1 contains JunD, FosB, and ATF1. Furthermore, JunD and FosB were able to activate FER-1 enhancer activity by transient cotransfection with ferritin H-chloramphenicol acetyltransferase reporter constructs. This augmented enhancer activity was inhibited by E1A. In addition, we have defined the minimal sequence in the dyad element of FER-1 required for protein interaction. This was determined to be a C-rich sequence to which Sp1 and Sp3 bind. Experiments with recombinant proteins indicate that members of both transcription factor families simultaneously bind FER-1. Taken together, these results elucidate molecular mechanisms involved in the transcriptional regulation of a pivotal gene in iron metabolism and provide insights into the contribution of the Sp1 family to the activation of AP1-dependent enhancers.

Overlapping Sp1 and AP2 binding sites in a promoter element of the lens-specific MIP gene

The MIP gene, the founder of the MIP family of channel proteins, is specifically expressed in fiber cells of the ocular lens and expression is regulated temporally and spatially during development. We previously found that a DNA fragment containing 253 bp of 5'-flanking sequence and 42 bp of exon 1 of the human MIP gene contains regulatory elements responsible for lens-specific expression of the MIP gene. In this report we have analyzed the function of overlapping Sp1 and AP2 binding sites present in the MIP promoter. Using DNase I footprinting analysis we found that purified Sp1 and AP2 transcription factors interact with several domains of the human MIP promoter sequence -253/+42. Furthermore, addition of purified Sp1 to Drosophila nuclear extracts activates in vitro transcription from the MIP promoter -253/+42. This promoter activity is competed by oligonucleotides containing domains footprinted with Sp1. Using promoter-reporter gene ( CAT ) constructs we found that the sequence -39/-70 contains a cis regulatory element essential for promoter activity in transient assays in lens cells. EMSA analysis showed that lens nuclear extracts contain factors that bind to the MIP 5'-flanking sequence containing overlapping Sp1 and AP2 binding domains at positions -37/-65. Supershift experiments with lens nuclear extracts indicated that Sp3 is also able to interact with this regulatory element, suggesting that Sp1 and Sp3 may be involved in regulation of transcription of the MIP gene in the lens.

Characteristics of the intron involvement in the mitogen-induced expression of Zfp-36

Zfp-36, the gene encoding the putative zinc finger protein tristetraprolin (TTP), is rapidly induced in fibroblasts by a variety of growth factors. Recent gene knockout experiments have shown that TTP-deficient mice developed arthritis, cachexia, and autoimmunity, all apparently mediated by an excess of tumor necrosis factor alpha. We recently showed that full serum inducibility of Zfp-36 requires elements in the promoter; in addition, removal of the single intron strikingly inhibited serum-induced TTP expression. We show here that replacement of the intron with unrelated sequences, or removal of 95% of the intron but retention of the splice sites, each resulted in the maintenance of approximately 45 and 19%, respectively, of full serum-induced expression. In addition, deletion of intron sequences base pairs 601-655 decreased the serum-induced expression of TTP by 65%. Sequence base pairs 618-626 bound specifically to the transcription factor Sp1; mutation of this binding motif decreased TTP expression by 70%, suggesting that Sp1 binding to this motif contributes to serum induction of Zfp-36. We conclude that full serum-induced expression of Zfp-36 depends on the activation of conventional promoter elements as well as elements in the single intron, and that the presence per se of the intron in its natural location also contributes significantly to the regulated expression of this gene.

Stimulation of the preprothyrotropin-releasing hormone gene by epidermal growth factor

Regulation of the expression of the prepro-TRH (ppTRH) gene by epidermal growth factor (EGF) was investigated. The i.p. injection of EGF significantly stimulated hypothalamic ppTRH messenger RNA levels in rats. To clarify whether this stimulatory effect of EGF could be exerted at the level of gene transcription, the 5'-flanking region (-1893/+127) of the mouse ppTRH gene fused to a luciferase reporter gene was transiently transfected into pituitary GH4C1 cells, and the effect of EGF on gene transcription was measured by a luciferase assay. EGF stimulated ppTRH gene promoter activity in a time- and dose-dependent manner. Deletion analysis revealed that two different regions of the promoter, between -254 and -218 [EGF response element-1 (EGFRE1)] and between -130 and -84 (EGFRE2) were required for full stimulation by EGF. The two EGFREs possessed putative binding sequences for the transcription factor Sp1, and they functioned cooperatively in heterologous promoters. Nuclear extracts from GH4C1 cells specifically bound those two EGFREs in gel retardation assays. Two protein-DNA complexes were found on EGFRE1, whereas four complexes were observed on EGFRE2. Although the binding of nuclear extracts to EGFRE1 was competed for by the consensus Sp1 binding sequence, the complexes on EGFRE1 were not supershifted by an Sp1 antibody. Formation of the slower migrating protein complex on EGFRE1 was prevented by EDTA, suggesting that one of the EGFRE1-binding proteins might be an Sp1-related zinc finger protein. Competition and supershift experiments demonstrated that the EGFRE2-binding protein showing that the slowest migration possessed a characteristic similar to that of Sp1. Selective mutations of the Sp1-binding site in EGFRE2 markedly diminished the EGF-induced stimulation. These results suggest that EGF may function as a positive regulator of ppTRH gene expression, and that the stimulatory effect may be mediated through a cooperative interaction between Sp1 or Sp1-related proteins and additional factors that bind to two separate DNA regions.

The testis-specific histone H1t gene is strongly repressed by a G/C-rich region just downstream of the TATA Box

H1t is a testis-specific histone 1 variant restricted to the male germ line and expressed only in pachytene spermatocytes. Understanding the regulation of the H1t gene is an interesting challenge as its promoter shares all of the recognized control elements of standard somatic H1 genes, yet H1t is not expressed in somatic or in early spermatogenic cells. To investigate the mechanism of this apparent repression, we exchanged three promoter subregions between H1t and a major somatic H1 gene (H1d) by introduction of suitable restriction sites just 5' of the TATA box and 3' of the conserved H1 AC box. Hybrid promoters were joined to a lacZ reporter gene and assayed by transient transfection in NIH3T3 fibroblasts. In this system the wild type H1d promoter was 20-fold stronger than the H1t promoter. Much of this difference in activity was traced to inhibitory sequences immediately downstream of the TATA box in H1t, although sequences upstream of the H1t AC box and within the H1t 5'-untranslated region played some role as well. A series of deletions and short oligonucleotide mutations scanned across the region between the TATA box and cap site identified two tracts of C (GC box 2) as the inhibitory sequences. While both Sp1 and Sp3 bind to this region weakly in vitro, they are unlikely to be responsible for the inhibitory effect of GC box 2, and additional binding proteins (CTB-4 and CTB-5) were identified by electrophoretic mobility shift assays as better candidates for mediating the repressive effect. When repression of the H1t promoter was relieved by mutation of GC box 2, additional mutations introduced into GC box 1 upstream of the CAAT box led to a large decrease in activity, indicating that these two G/C-rich elements have opposite effects on promoter activity.

Transcriptional regulation of the rat poly(ADP-ribose) polymerase gene by Sp1

Expression of the gene encoding poly(ADP-ribose) polymerase (PARP), although ubiquitous, nevertheless varies substantially between tissues. We have recently shown that Sp1 binds five distinct target sequences (US-1 and F1-F4) in the rat PARP (rPARP) gene promoter. Here we used deletion analyses and site-directed mutagenesis to address the regulatory function played by these Sp1 sites on the basal transcriptional activity directed by the rPARP promoter. Transfection experiments revealed that the most proximal Sp1 site is insufficient by itself to direct any promoter activity. In addition, a weak negative regulatory element was identified between positions -101 and -60. The rPARP promoter directed high levels of chloramphenicol acetyltransferase activity in Jurkat T-lymphoblastoid and Ltk- fibroblast cells but only moderate levels in pituitary GH4C1 and liver HTC cells, correlating with the amounts of PARP detected in these cells by western blot analysis. However, the reduced promoter efficiency in HTC and GH4C1 cells did not result from the lack of Sp1 activity in these cells but suggested that yet uncharacterized regulatory proteins might turn off PARP gene expression by binding negative regulatory elements from the rPARP promoter. Similarly, site-directed mutagenesis on the three most proximal Sp1 elements suggested the influence exerted by Sp1 on the rPARP promoter activity to vary substantially between cell types. It also provided evidence for a basal rPARP promoter activity driven through the recognition of unidentified cis-acting elements by transcription factors other than Sp1.

Transcriptional regulation by the Sp family proteins

Sp1 is one of the very first cellular transcription factors to be identified and cloned in virtue of its binding to a G-rich motif in the SV40 early promoter. Sp1 protein binds to the G-rich sequences present in a variety of cellular and viral promoters and stimulates their transcriptional activity. Recently, a number of other GC and/or GT box-binding factors homologous to Sp1 have been isolated, namely Sp2, Sp3 and Sp4, and the two more distantly related factors, BTEB and BTEB2. The discovery of this family highlights a previously unknown level of complexity of transcriptional regulation of promoters containing GC and/or GT box motifs. This review focuses primarily on strategies aimed to elucidate the transcription properties of the Sp1-like factors and discusses the experimental problems inherent in the attempt to define their respective functions.

Follicle stimulating hormone-regulated expression of serum/glucocorticoid-inducible kinase in rat ovarian granulosa cells: a functional role for the Sp1 family in promoter activity

Recently, a family of novel, serine/threonine protein kinases has been identified. One of these transcriptionally inducible, immediate-early genes encodes serum/glucocorticoid inducible-protein kinase, sgk. By in situ hybridization, we show that sgk expression in the rat ovary is selectively localized to granulosa cells. In culture, FSH or forskolin, activators of the protein kinase A (PKA) pathway, rapidly (2 h) and transiently increased sgk mRNA levels in undifferentiated granulosa cells. Sgk mRNA exhibited a biphasic expression pattern, with maximal levels observed at 48 h of FSH/forskolin as granulosa cells differentiate to the preovulatory phenotype. Deletion analyses using sgk promoter-reporter constructs (-4.0 kb to -35 bp) identified a region between -63 and -43 bp that mediated FSH and forskolin-responsive transcription in undifferentiated and differentiated granulosa cells. This G/C-rich region 1) conferred both basal and inducible transcription to the minimal -35 sgk promoter chloramphenicol acetyltransferase reporter construct, 2) specifically bound Sp1 and Sp3 present in granulosa cell extracts, and 3) bound recombinant Sp1. Mutation of 2 bp in this region not only prevented Sp1 and Sp3 binding, but also abolished the PKA-mediated transactivation observed when using the wild type construct. Sp1 and Sp3 DNA-binding activity and protein levels did not change significantly during sgk induction. Collectively, these data indicate that Sp1/Sp3 transactivation of the sgk promoter likely involves regulated, phosphorylation-dependent interaction with other factors. Thus the novel, biphasic induction of sgk that correlates with granulosa cell progression from proliferation to differentiation appears to involve sequential, coordinated actions of FSH, PKA, and transcription factors, including Sp1 and Sp3.

Characterization of multiple enhancer regions upstream of the apolipoprotein(a) gene

Plasma concentrations of the atherogenic lipoprotein(a) (Lp(a)) are predominantly determined by inherited sequences within or closely linked to the apolipoprotein(a) gene locus. Much of the interindividual variability in Lp(a) levels is likely to originate at the level of apo(a) gene transcription. However, the liver-specific apo(a) basal promoter is extremely weak and does not exhibit common functional variations that affect plasma Lp(a) concentrations. In a search for additional apo(a) gene control elements, we have identified two fragments with enhancer activity within the 40-kilobase pair apo(a)-plasminogen intergenic region that coincide with DNase I-hypersensitive sites (DHII and DHIII) observed in liver chromatin of mice expressing a human apo(a) transgene. Neither enhancer exhibits tissue specificity. DHIII activity was mapped to a 600-base pair fragment containing nine DNase I-protected elements (footprints) that stimulates luciferase expression from the apo(a) promoter 10-15-fold in HepG2 cells. Binding of the ubiquitous transcription factor Sp1 plays a major role in the function of this enhancer, but no single site was indispensable for activity. DHIII comprises part of the regulatory region of an inactive long interspersed nucleotide element 1 retrotransposon, raising the possibility that retrotransposon insertion can influence the regulation of adjacent genes. DHII enhancer activity was localized to a 180-base pair fragment that stimulates transcription from the apo(a) promoter 4-8-fold in HepG2 cells. Mutations within an Sp1 site or either of two elements composed of direct repeats of the nuclear hormone receptor half-site AGGTCA in this sequence completely abolished enhancer function. Both nuclear hormone receptor elements were shown to bind peroxisome proliferator-activated receptors and other members of the nuclear receptor family, suggesting that this enhancer may mediate drug and hormone responsiveness.

Cloning and characterization of murine glial cell-derived neurotrophic factor inducible transcription factor (MGIF)

The potent neurotrophic factor glial cell-derived neurotrophic factor (GDNF) is a distant member of the transforming growth factor-beta (TGF-beta) superfamily of proteins. We report a transcription factor that is the first nuclear protein known to be induced by GDNF, thus designated murine GDNF inducible factor (mGIF). The cDNA was cloned in the course of investigating transcription factors that bind to Sp1 consensus sequences, using the in situ filter detection method, and it was found to encode a protein having the same C2-H2 zinc finger motif as Sp1. Sequence analysis indicated that mGIF is homologous to the human TGF-beta inducible early gene (TIEG) and human early growth response gene-alpha (EGR-alpha). mGIF is widely distributed in the adult mouse with high mRNA levels in kidney, lung, brain, liver, heart, and testis. In the adult brain, mGIF is abundantly expressed in hippocampus, cerebral cortex, cerebellum, and amygdala with lower amounts in striatum, nucleus accumbens, olfactory tubercle, thalamus, and substantia nigra. During development, mGIF mRNA also has a wide distribution, including in cerebral cortex, cerebellar primordium, kidney, intestine, liver, and lung. GDNF induces the expression of mGIF rapidly and transiently both in a neuroblastoma cell line and in primary cultures of rat embryonic cortical neurons. Co-transfection of the Drosophila SL2 cells using mGIF expression plasmid and reporter constructs having Sp1 binding sites indicated that mGIF represses transcription from a TATA-containing as well as from a TATA-less promoter. These observations suggest that the zinc finger transcription factor mGIF could be important in mediating some of the biological effects of GDNF.

Transcriptional regulation of the human erythroid 5-aminolevulinate synthase gene. Identification of promoter elements and role of regulatory proteins

We have characterized the 5'-flanking region of the human erythroid-specific 5-amino levulinate synthase (ALAS) gene (the ALAS2 gene) and shown that the first 300 base pairs of promoter sequence gives maximal expression in erythroid cells. Transcription factor binding sites clustered within this promoter sequence include GATA motifs and CACCC boxes, critical regulatory sequences of many erythroid cell-expressed genes. GATA sites at -126/-121 (on the noncoding strand) and -102/-97 were each recognized by GATA-1 protein in vitro using erythroid cell nuclear extracts. Promoter mutagenesis and transient expression assays in erythroid cells established that both GATA-1 binding sites were functional and exogenously expressed GATA-1 increased promoter activity through these sites in transactivation experiments. A noncanonical TATA sequence at the expected TATA box location (-30/-23) bound GATA-1- or TATA-binding protein (TBP) in vitro. Conversion of this sequence to a canonical TATA box reduced expression in erythroid cells, suggesting a specific role for GATA-1 at this site. However, expression was also markedly reduced when the -30/-23 sequence was converted to a consensus GATA-1 sequence (that did not bind TBP in vitro), suggesting that a functional interaction of both factors with this sequence is important. A sequence comprising two overlapping CACCC boxes at -59/-48 (on the noncoding strand) was demonstrated by mutagenesis to be functionally important. This CACCC sequence bound Sp1, erythroid Krüppel-like factor, and basic Krüppel-like factor in vitro, while in transactivation experiments erythroid Krüppel-like factor activated ALAS2 promoter expression through this sequence. A sequence at -49/-39 with a 9/11 match to the consensus for the erythroid specific factor NF-E2 was not functional. Promoter constructs with 5'-flanking sequence from 293 base pairs to 10.3 kilobase pairs expressed efficiently in COS-1 cells as well as in erythroid cells, indicating that an enhancer sequence located elsewhere or native chromatin structure may be required for the tissue-restricted expression of the gene in vivo.

The locus of Epstein-Barr virus terminal repeat processing is bound with enhanced affinity by Sp1 and Sp3

EBV DNA contains G-rich, repeat regions that are involved in rearrangement and recombination events including terminal repeat (TR) processing and the EBNA-2 deletion in the EBV strain P3HR-1. Cellular proteins, called terminal or tandem repeat binding proteins (TRBPs), recognize sequences at the junctions of these recombination events. In this study, using antibody supershift assays and expression of recombinant proteins, we show that Sp1 and Sp3 are the sequence-specific components of TRBP and that Ku is the nonspecific binding component. Sp1 binds other recombinogenic regions of EBV DNA, but Sp3 does not bind to the large internal repeat. The sequence GGGGTGGGG, a low affinity site for Sp1 and Sp3, is the minimal binding site within terminal repeat binding site 1 (TRBS1). However, 3' flanking sequences in the sequence GGGGTGGGGCATGGGG augment binding of Sp1 and Sp3 so that their affinity of binding is increased approximately twofold relative to a classical high-affinity Sp1 site. EBV lytic cycle induction does not alter the abundance or binding activity of any of the three identified components of TRBP. Sp1 and Sp3 may act in trans to promote EBV terminal repeat processing and possibly other viral and cellular recombination events.

Sp1 and Sp3 regulate expression of the neuronal nicotinic acetylcholine receptor beta4 subunit gene

Neuronal nicotinic acetylcholine receptors play important roles in signal transduction within the nervous system. The receptors exist in a variety of functionally distinct subtypes that are determined by their subunit structures. The subunits are encoded by 11 genes, alpha2-alpha9 and beta2-beta4. Three of the genes, alpha3, alpha5, and beta4, are tightly clustered, and their encoded proteins make up the predominant receptor subtype in the peripheral nervous system. The tight linkage of the genes suggests there may be a common regulatory mechanism underlying their expression. However, although their expression patterns significantly overlap, they are not identical, indicating that independent regulatory mechanisms must also exist. Our studies have focused upon the gene encoding the beta4 subunit for which we have identified several transcriptional regulatory elements. One of these elements, E2, specifically interacts with the general transcription factor Sp1. Here we show that another member of the Sp family of factors, Sp3, can specifically interact with E2 whereas two other members, Sp2 and Sp4, cannot. Co-transfection experiments indicate that Sp3 can transactivate a beta4 promoter/reporter gene construct and, furthermore, that Sp1 and Sp3 can transactivate the beta4 reporter construct synergistically. The transactivation is dependent upon an intact E2 and may involve direct interactions between Sp1 and Sp3.

Egr transcription factors in the nervous system

The Egr proteins, Egr-1, Egr-2, Egr-3 and Egr-4, are closely related members of a subclass of immediate early gene-encoded, inducible transcription factors. They share a highly homologous DNA-binding domain which recognises an identical DNA response element. In addition, they have several less-well conserved structural features in common. As immediate early proteins, the Egr transcription factors are rapidly induced by diverse extracellular stimuli within the nervous system in a discretely controlled manner. The basal expression of the Egr proteins in the developing and adult rat brain and the induction of Egr proteins by neurotransmitter analogue stimulation, physiological mimetic and brain injury paradigms is reviewed. We review evidence indicating that Egr proteins are subject to tight differential control through diverse mechanisms at several levels of regulation. These include transcriptional, translational and post-translational (including glycosylation, phosphorylation and redox) mechanisms and protein-protein interaction. Ultimately the differentially co-ordinated Egr response may lead to discrete effects on target gene expression. Some of the known target genes of Egr proteins and functions of the Egr proteins in different cell types are also highlighted. Future directions for research into the control and function of the different Egr proteins are also explored.

Two Sp sites are important cis elements regulating the upstream promoter region of the gene for rat type I hexokinase

Multiple transcriptional start sites have been identified for the gene encoding the rat Type I isozyme of hexokinase (White, J.A., Liu, W., and Wilson, J. E., Arch. Biochem. Biophys. 335, 161-172, 1996); these are clustered at positions approximately -460, -300, and -100 relative to the translational start codon (ATG, with A being +1). PC12 cells and H9c2 cells were transfected with luciferase reporter constructs containing genomic sequence between positions -3366 and -171. Marked (85%) decrease in promoter activity was associated with deletion of sequence between -742 and -516. In DNase I footprinting experiments, two regions, called P1 (-552 to -529) and P2 (-480 to -458) boxes, were protected by proteins present in nuclear extracts from PC12 cells. Mutation or deletion of the P2 box had no effect on promoter activity; protection in this region, which includes the most upstream cluster of transcriptional start sites, is attributed to binding of RNA polymerase II or associated factors. In contrast, mutations or deletions in the P1 box had markedly detrimental effects on promoter activity and on binding of proteins in PC12 cell nuclear extracts. Maintenance of a consensus Sp1 binding site centrally located in the P1 box was critical for both promoter activity and binding. A second Sp1 site (-570), just upstream from the P1 box, was also shown to be functionally important but no protection of this region was detected in footprinting experiments, presumably reflecting lower affinity at this site under the conditions used. Supershift experiments demonstrated the involvement of Sp1, Sp3, and Sp4 in formation of complexes with the P1 box region and implicate these transcription factors in regulating promoter activity associated with this region. Another series of reporter constructs, including sequence between -171 and -1, permitted detection of an additional promoter activity downstream from -364. While not yet extensively characterized, it is already evident that the cis elements influencing the downstream promoter activity are distinct from the Sp factors determined to be important in expression from the upstream promoter region.

The keratinocyte-specific Epstein-Barr virus ED-L2 promoter is regulated by phorbol 12-myristate 13-acetate through two cis-regulatory elements containing E-box and Krüppel-like factor motifs

We previously employed 782 base pairs of the Epstein-Barr virus ED-L2 early lytic cycle promoter in a transgenic mouse model to target cyclin D1 to the stratified squamous epithelium of the tongue and esophagus. This promoter is located 5' to the transcriptional start site of a short open reading frame BNLF-2A and is immediately 3' to the BNLF-1 (LMP-1 oncogene) open reading frame. We studied transcriptional regulation of the ED-L2 promoter by phorbol 12-myristate 13-acetate (PMA) as a means of understanding the tissue specificity of this promoter. The transcriptional activity of the ED-L2 promoter was stimulated 40-fold by PMA and could be blocked with the compound H7 through antagonism of protein kinase C. 5' deletion analysis of the 782-base pair promoter demonstrated that the sequences necessary for PMA-stimulated trans-activation were located in two separate cis-regulatory regions of the promoter: -187 to -164 and -144 to -114 base pairs from the transcription start site of BNLF-2A. Importantly, mutation of critical base pairs in each region was sufficient to abolish PMA-stimulated trans-activation in the native ED-L2 promoter. Region -187 to -164 contains a CACCTG (E-box) motif, and region -144 to -114 contains a CACACCC motif. Both of these motifs are necessary for trans-activation by PMA. These regions do not, however, demonstrate enhancer characteristics when tested in a heterologous minimal promoter system. Variations of the CACACCC motif are found in other keratinocyte-specific promoters, as well as in the DNA binding motifs of the Krüppel-like family of transcription factors. Electrophoretic mobility shift assays with specific competitors and factor-specific antibody supershift assays demonstrated that one complex binding the -187 to -164 region containing the CACCTG nucleotides has characteristics of the helix-loop-helix protein upstream stimulatory factor, whereas a factor binding the CACACCC motif may be a member of the Krüppel-like family. These experiments show how ubiquitous and tissue-specific transcription factors induced by PMA regulate the ED-L2 promoter in squamous epithelial cells.

Sp3 mediates transcriptional activation of the leukocyte integrin genes CD11C and CD11B and cooperates with c-Jun to activate CD11C

The leukocyte integrin genes CD11c and CD11b are expressed predominately in myelomonocytic cells. In previous experiments, the -70 to -65 and -121 to -103 regions of the CD11c promoter and the -66 to -59 region of the CD11b promoter were shown to be essential for Sp1-mediated activation of these genes. In vivo genomic footprinting had also revealed cell-specific binding of protein, presumably Sp1, to these regions. In this study, electrophoretic mobility shift analysis showed that the Sp1-related factor, Sp3, also binds at or near these same regions. Cotransfection of Sp3 along with CD11c promoter-luciferase constructs into Sp-deficient Drosophila Schneider 2 cells showed that Sp3 could activate the CD11c promoter. Deletion of both the -70 to -65 and -121 to -103 regions of the CD11c promoter resulted in the loss of activation by Sp3. Both sites showed activation by Sp3; however, the -70 to -65 region was more responsive to Sp3 than to Sp1. Similar transfection analysis of the -66 to -59 region of the CD11b promoter showed Sp3-dependent expression. Further, cotransfection analysis in Drosophila cells showed that Sp3, as was previously shown for Sp1, also synergizes with c-Jun to activate CD11c. Antisense experiments that knocked out endogenous Sp3 expression in the myelomocytic cell line, HL60, revealed that Sp3 participates in activation of the CD11c and CD11b promoters in vivo.

Collaborative roles for c-Jun N-terminal kinase, c-Jun, serum response factor, and Sp1 in calcium-regulated myocardial gene expression

Electrical stimulation of contractions (pacing) of primary neonatal rat ventricular myocytes increases intracellular calcium and activates a hypertrophic growth program that includes expression of the cardiac-specific gene, atrial natriuretic factor (ANF). To investigate the mechanism whereby pacing increases ANF, pacing was tested for its ability to regulate mitogen-activated protein kinase family members, ANF promoter activity, and the trans-activation domain of the transcription factor, Sp1. Pacing and the calcium channel agonist BAYK 8644 activated c-Jun N-terminal kinase (JNK) but not extracellular signal-regulated kinase. Pacing stimulated ANF-promoter activity approximately 10-fold. Furthermore, transfection with an expression vector for c-Jun, a substrate for JNK, also activated the ANF promoter, and the combination of pacing and c-Jun was synergystic, consistent with roles for JNK and c-Jun in calcium-activated ANF expression. Proximal serum response factor and Sp1 binding sites were required for the effects of pacing or c-Jun on the ANF promoter. Pacing and c-Jun activated a GAL4-Sp1 fusion protein by 3- and 12-fold, respectively, whereas the two stimuli together activated GAL4-Sp1 synergistically, similar to their effect on the ANF promoter. Transfection with an expression vector for c-Fos inhibited the effects of c-Jun, suggesting that c-Jun acts independently of AP-1. These results demonstrate an interaction between c-Jun and Sp1 and are consistent with a novel mechanism of calcium-mediated transcriptional activation involving the collaborative actions of JNK, c-Jun, serum response factor, and Sp1.

The 5'-upstream region of the rat phospholipase C-beta 3 gene contains two critical Sp1 sites and an HIV Inr-like element

The 5'-upstream region of the rat phospholipase C-beta 3 gene (PLC-beta 3) has been cloned and characterized. Sequence analysis of the 5'-upstream region showed that it contains a GC-rich region (-166 to +1: 79%) and multiple binding sites for the transcription factors Sp1, AP-1 and AP-2, but does not contain a canonical TATA box. Primer extension analysis of total RNA isolated from rat glial cell C6Bul revealed that single transcription start point (tsp) is located at an initiator (Inr) element similar to that found in the HIV promoter. Gel mobility shift and competitive mobility shift assays indicated that this Inr element forms a DNA-protein complex with the HIV Inr-binding protein, LBP-1/CP2 or a homologue. In order to localize functional elements of the 5'-upstream region of the rat PLC-beta 3 gene, 5'-deletion fragments were cloned into a chloramphenicol acetyltransferase (CAT) reporter vector. Transient transfection analyses of the 5'-deletion mutants identified a crucial promoter element located at -128 to -14. Supershift mobility assays, site-directed mutagenesis and DNase I footprints indicated that Sp1 binds to three GC boxes within the sequence between -128 and -14 of the PLC-beta 3 promoter. Transient transfection analyses of promoter constructs containing site-specific mutation(s) of these three GC boxes demonstrated that two GC boxes, located proximal to the tsp, are important elements for normal promoter activity.

Sp3 is a transcriptional activator of the human alpha2(I) collagen gene

Sp3 is an ubiquitously expressed transcription factor, closely related to Sp1 but, unlike Sp1, it often functions as a transcriptional repressor. In this study we investigated the role of Sp3 in regulating the transcription of the human alpha2(I) collagen gene. We show that Sp1 and Sp3 specifically bind to three of the previously characterized cis -elements in this promoter, including two positive cis-elements between -303 and -271 and -128 and -123, and a repressor site between -164 and -159, but do not bind to the fourth cis-element bound by CBF. Functional analyses of Sp3 and Sp1 in Drosophila cells indicate that each protein transactivates the human alpha2(I) collagen promoter with equal potency and, when tested together, have an additive effect on the promoter activity. Furthermore, in vitro transcription assays demonstrate that both Sp1 and Sp3 are capable of supporting transcription from the collagen promoter independently of each other. However, when activities of both Sp1 and Sp3 are blocked with specific antibodies, in vitro transcription from this promoter is almost completely abolished. The results of this study demonstrate that Sp3 is as potent an activator of the human alpha2(I) collagen promoter as is Sp1 and that a transcriptional activity of the human alpha2(I) promoter is dependent on both proteins.

The interferon gamma (IFN-gamma) receptor: a paradigm for the multichain cytokine receptor

With the purification and cloning of the interferon gamma (IFN-gamma) receptor chains the mechanism of IFN-gamma action and the resultant signal transduction events were delineated in remarkable detail. The interferon gamma (IFN-gamma) receptor complex consists of two chains: IFN-gammaR1, the ligand-binding chain, and IFN-gammaR2, the accessory chain. Binding of IFN-gamma causes oligomerization of the two IFN-gamma receptor subunits, IFN-gammaR1 and IFN-gammaR2, which initiates the signal transduction events: activation of Jak1 and Jak2 receptor associated protein tyrosine kinases, phosphorylation of the IFN-gammaR1 intracellular domain on Tyr440 followed by phosphorylation and activation of Stat1alpha, the latent transcriptional factor. With all these steps established, the IFN-gamma receptor complex has provided the basic model for understanding the receptors for other members of the family of class II cytokine receptors.

Binding of SP1 to the immediate-early protein-responsive element of the human cytomegalovirus DNA polymerase promoter

Human cytomegalovirus (HCMV), a member of the herpesvirus family of DNA viruses, encodes two major immediate-early (IE) transcription factors, IE72 and IE86, that are important for regulated expression of the viral genome. The purpose of this study was to identify the host cellular components required for regulation of the HCMV DNA polymerase promoter (UL54) by HCMV IE proteins. Extensive mutagenesis defined a DNA element located between -54 and -43 relative to the transcription start site that was required for both basal transcriptional activity and transactivation by viral IE proteins. A single copy of the UL54 -54/-43 sequence enhanced the responsiveness of a heterologous minimal promoter to HCMV IE proteins. Fractionation of extracts prepared from uninfected cells led to the isolation of two cellular proteins with apparent molecular masses of 95 and 105 kDa that bound specifically to the UL54 -54/-43 element. Biochemical and immunochemical analyses identified this protein as the transcription factor SP1. Although initial inspection of the UL54 -54/-43 sequence did not predict an SP1 binding site, subsequent analyses indicated that it is indeed a nonconsensus GC box. We propose that SP1 is required to direct basal levels of promoter activity and that SP1-regulated transcription complexes allow the entry of HCMV IE proteins into the transcription cycle.

Nerve growth factor induces transcription of the p21 WAF1/CIP1 and cyclin D1 genes in PC12 cells by activating the Sp1 transcription factor

The PC12 pheochromocytoma cell line responds to nerve growth factor (NGF) by gradually exiting from the cell cycle and differentiating to a sympathetic neuronal phenotype. We have shown previously () that NGF induces the expression of the p21 WAF1/CIP1/Sdi1 (p21) cyclin-dependent kinase (Cdk) inhibitor protein and the G1 phase cyclin, cyclin D1. In this report, we show that induction is at the level of transcription and that the DNA elements in both promoters that are required for NGF-specific induction are clusters of binding sites for the Sp1 transcription factor. NGF also induced a synthetic promoter with repeated Sp1 sites linked to a core promoter, and a plasmid regulated by a chimeric transactivator in which the Gal4 DNA binding domain is fused to the Sp1 transactivation domain, indicating that this transactivation domain is regulated by NGF. Epidermal growth factor, which is a weak mitogen for PC12, failed to induce any of these promoter constructs. We consider a model in which the PC12 cell cycle is arrested as p21 accumulates and attains inhibitory levels relative to Cdk/cyclin complexes. Sustained activation of p21 expression is proposed to be a distinguishing feature of the activity of NGF that contributes to PC12 growth arrest during differentiation

Functional interactions between the estrogen receptor and the transcription activator Sp1 regulate the estrogen-dependent transcriptional activity of the vitellogenin A1 io promoter

Two distinct, TATA box-containing promoters regulate the transcriptional activity of the Xenopus vitellogenin A1 gene. These two promoters are of different strength and are separated by 1.8 kilobase pairs of untranslated sequence. Estrogen receptor (ER) and its ligand, 17beta-estradiol, induce the activity of both promoters. The estrogen response elements (EREs) are located proximal to the downstream i promoter while no ERE-like sequences have been identified in the vicinity of the upstream io promoter. We show here, that transcriptional activity of the upstream io promoter is Sp1-dependent. Moreover, we demonstrate that estrogen inducibility of the io promoter results from functional interactions between the io bound Sp1 and the ER bound at the proximity of i. Functional interactions between Sp1 and ER do not require the presence of a TATA box for transcriptional activation, as is demonstrated using the acyl-CoA oxidase promoter. The relative positions that ER and Sp1 occupy with respect to the initiation site determines whether these two transcription activators can synergize for transcription initiation.

Transcriptional regulatory region for expression of the rat ATP citrate-lyase gene

We previously mapped the sequences responsive to insulin/glucose stimulation and polyunsaturated fatty-acid suppression in the proximal promoter region between positions -104 and -20 of the ATP citrate-lyase (ACL) gene [Fukuda, H., Iritani, N., Katsurada, A. & Noguchi, T. (1996) FEBS Lett. 380, 204-207]. To investigate further the regulatory DNA sequences required for stimulation and suppression of this gene, primary cultured hepatocytes were transfected with plasmids containing the 5'-flanking sequences of the rat ACL gene fused to the chloramphenicol acetyltransferase (CAT) gene. When two copies of the sequences spanning -64 to -41 (linked to ACLcat20) were used for transfection, CAT activity significantly increased in response to insulin/glucose treatment. This increase was inhibited by addition of polyunsaturated fatty acid. Mutational analysis of this region showed that sequences between -55 and -51 are essential for recognition and interaction with trans-acting factors. Gel mobility shift assays using the sequence from -64 to -41 as a probe revealed nuclear factor(s) from rat liver that specifically complexed with the sequences. In addition, by antibody supershift assays, we have detected the binding of the transcriptional factor Sp1 at the G+C-rich region located within -64 to -41 of the ACL promoter. On the other hand, the formations of DNA-protein complexes with Sp1 binding site or ACL(-64 to -41) were decreased in rats fed a high-carbohydrate diet in comparison with those in rats fasted or fed a polyunsaturated fatty-acid-rich diet. Cotransfection studies in rat hepatocytes, with the Sp1 expression vector and ACLcat constructs, showed the inactivation of the promoter. These results demonstrated that the region from -64 to -41 of the ACL gene was responsible for stimulation due to insulin/glucose, the stimulation was suppressed by polyunsaturated fatty acid, and Sp1 may be involved in the regulation.

A CACCC box-like cis-regulatory element of the Epstein-Barr virus ED-L2 promoter interacts with a novel transcriptional factor in tissue-specific squamous epithelia

The Epstein-Barr (EBV) virus induces a lytic state after infecting epithelial cells. Subsequently, there is infection of B lymphocytes with two types of cycles, latent and lytic. Apart from linkage of the EBV latent membrane protein-1 (LMP-1) with benign and malignant conditions of squamous epithelial cells, little is known about other EBV gene products that may be important in these processes as well as cellular transcriptional factors that regulate EBV gene expression in these epithelial cells. The EBV ED-L2 promoter, an early lytic cycle promoter, is located upstream of a transcription start site for a short open reading frame designated BNLF2 and just downstream of the BNLF1 (LMP-1) open reading frame. We have previously used the EBV ED-L2 promoter to target oncogenes in transgenic mice, resulting in tissue-specific expression in the tongue, esophagus, forestomach, and skin, all sharing stratifying squamous epithelia, alternatively called keratinocytes. In the present study, we have functionally dissected the ED-L2 promoter by making deletion constructs fused to the luciferase reporter gene with transient transfections into squamous and nonsquamous epithelial cell lines as well as B lymphocytes. A CACCC box-like cis-regulatory element has been identified that is located between -218 and -187 base pairs of the ED-L2 promoter that confers significant promoter activity only in squamous epithelial cells. This cis-regulatory element is active in a heterologous minimal herpes simplex virus thymidine kinase promoter reporter gene construct when transfected into squamous epithelial cells but not in nonsquamous epithelial cells. DNA gel mobility shift assays have led to the identification of DNA-protein complexes that bind the CACCC box-like element. One of these proteins is a novel transcriptional factor that is uniquely active in stratified squamous epithelial cells, designated as keratinocyte specific factor (KSF). KSF may be related to Sp1 but appears to be distinct from Sp1. In addition, KSF may interact with related or identical cis-regulatory elements found in human papillomavirus-11 E6 and cytokeratin K3 promoters that are active in keratinocytes. In aggregate, KSF may be important in the transcriptional regulation of viral and eukaryotic genes in keratinocytes.

Promoter of the Na,K-ATPase alpha3 subunit gene is composed of cis elements to which NF-Y and Sp1/Sp3 bind in rat cardiocytes

Na,K-ATPase alpha subunit has three isoforms whose expression is regulated developmentally and hormonally. Na,K-ATPase alpha3 subunit gene (Atpla3) is expressed only in brain and neonatal heart in a rat. The purpose of this study is to analyze cis-acting elements and trans-acting factors regulating the transcription of Atpla3 in cultured neonatal rat cardiocytes. Transient transfection assays with Atpla3-luciferase chimeric construct and a series of 5' sequential deletion mutations revealed the existence of positive regulatory elements from -74 to -59 and from -59 to -39. A factor was identified to bind across -59 by gel retardation assay. Methylation interference and DNase I footprinting analyses revealed the binding region from -74 to -53 (positive regulatory element (PRE) 1). The binding factor was identified to be NF-Y by gel retardation assay using specific antibody. Gel retardation and methylation interference analyses revealed that factors bind to two other elements from -54 to -43 (PRE2) and from -25 to -13 (PRE3). The binding factors were identified to be Sp1/Sp3 using specific antibodies. The functions of above-mentioned three elements were examined by transient transfection assay with various combinations of mutations. They all regulated the transcription positively and a synergistic enhancement of it was observed. Roles of NF-Y in the transcriptional activation and synergy are discussed.

Oct-1, silencer sequence, and GC box regulate thyroid hormone receptor beta1 promoter

Thyroid hormone, acting through thyroid hormone receptors (TRs), plays a crucial role in brain development and its insufficiency results in irreversible brain damage. TR alpha mRNA is expressed continuously from early embryonic stages, but the level of TR beta1 mRNA in brain is more abundant in adult than in fetus. To identify important factors which regulate TR beta1 expression, we compared mouse fetal and adult brain nuclear extracts by DNase I footprinting and electrophoretic gel mobility shift assays (EMSA) of the TR beta1 promoter. We carried out transient transfection studies in COS 1 cells using the TR beta1 promoter fused to Luciferase gene, and used mutated promoter vectors and various expression vectors. In DNase I footprinting using the fragment -950 to -717, fetal brain nuclear extracts protected the areas -910 to -884 and -815 to -800 more than did adult extracts. In EMSA, proteins in fetal nuclear extracts bound to a silencer sequence (-924 to -916), GC box (-901 to -887), and E box (-810 to -805), more strongly than did proteins in adult brain extracts. The bands formed on GC box were not supershifted by Sp-1, Sp-2, Sp-3, Sp-4, EGR-1, or EGR-2 antibodies. Three bands were detected on the octamer binding site probe (-913 to -906) and one protein was supershifted by Oct-1 antibody. Adult brain extracts appear to contain more Oct-1 protein than do fetal extracts. The other two bands were more intense in fetal extracts than in adult extracts, but were not supershifted by either Oct-1 or Oct-2 antibodies. Mutation of the silencer response element, mutation of the GC box, and Oct-1 over expression in COS 1 cells increased TR beta1 promoter function as assayed by Luciferase reporter. Mutation of the octamer binding site, to which only Oct-1 bound in COS 1 cells, decreased Luciferase reporter activity. Thus the TR beta1 promoter was regulated negatively by the proteins bound to the silencer sequence and the GC box, and positively by Oct-1. Silencer and GC box binding proteins are more abundant in fetal brain, and Oct-1 is more abundant in adult brain. The results may be responsible for increased amounts of TR beta1 present in late fetal and adult brain.

The retinoblastoma protein: a master regulator of cell cycle, differentiation and apoptosis

The retinoblastoma susceptibility gene is a tumour suppressor and its product retinoblastoma protein (pRb) has been known for 10 years as a repressor of progression towards S phase. Its major activity was supposed to be sequestration or inactivation of the transcription factor E2F which is required for activation of S phase genes. However, within recent years growing evidence has been accumulating for a more general function of pRb at both the transcriptional level and the cellular level. pRb not only regulates the activity of certain protein-encoding genes but also the activity of RNA polymerase pol I and pol III transcription. This protein appears to be the major player in a regulatory circuit in the late G1 phase, the so-called restriction point. Moreover, it is involved in regulating an elusive switch point between cell cycle, differentiation and apoptosis. Here, it seems to cooperate with another major tumour suppressor, p53. Thus, pRb sits at the interface of the most important cell-regulatory processes and therefore deserves close attention by specialists from different fields of research. This review provides an introduction to the complex functions of pRb.