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

Hepatic expression of the tumor necrosis factor family member lymphotoxin-beta is regulated by interleukin (IL)-6 and IL-1beta: transcriptional control mechanisms in oval cells and hepatoma cell lines

BACKGROUND

Lymphotoxin-beta (LT-beta) plays an important role in inflammation and its promoter contains a functional nuclear factor-kappaB (NF-kappaB) element, rendering it a likely target of pro-inflammatory cytokines. Inflammatory cytokines play a central role in liver regeneration resulting from acute or chronic liver injury, with interleukin (IL)-6 signaling essential for liver regeneration induced by partial hepatectomy. In hepatic oval cells observed following chronic liver injury, LT-beta levels are upregulated, suggesting a link between LT-beta and liver regeneration.

RESULTS

The expression of LT-beta in hepatic oval cell and hepatocellular carcinoma cell lines was further investigated, along with its responsiveness to IL-6 and IL-1beta. Key regulatory cis-acting elements of the LT-beta promoter that mediate IL-6 responsiveness (Sp/BKLF, Ets, NF-kappaB and Egr-1/Sp1) and IL-1beta responsiveness (NF-kappaB and Ets) of hepatic LT-beta expression were identified. The novel binding of basic Kruppel-like factor (BKLF) proteins to an apparent composite Sp/BKLF site of the LT-beta promoter was shown to mediate IL-6 responsiveness. Binding of NF-kappaB p65/p50 heterodimers and Ets-related transcription factors to their respective sites mediates responsiveness to IL-1beta.

CONCLUSION

The identification of IL-6 and IL-1beta as activators of LT-beta supports their involvement in LT-beta signaling in liver regeneration associated with chronic liver damage.

Different roles of Sp family members in HIV-1 Tat-mediated manganese superoxide dismutase suppression in hepatocellular carcinoma cells

The expression of manganese superoxide dismutase (MnSOD) is regulated by agents associated with cancer development. It has been shown that infection with the human immunodeficiency virus type 1 (HIV-1) is associated with the development of liver cancer and that the transactivating transcriptional factor (Tat) of human HIV-1 reduces the expression of MnSOD in several cell types. However, the role of Tat in the expression of MnSOD in hepatocellular carcinoma is unknown. Furthermore, the precise mechanisms whereby Tat suppresses MnSOD expression in hepatocellular carcinoma cells remain unclear. In this report, we build on our original observations that Tat changes the distribution of Sp family members on the MnSOD promoter, which accounts for Tat-dependent changes in basal expression. In hepatic cells, Tat expression upregulates Sp1/Sp3, which play different roles in regulating MnSOD transcription. While overexpression of Sp1 stimulates, overexpression of Sp3 represses transcriptional activity. The transcription repression effect of Sp3 is not due to Sp3 competing for the binding site with Sp1 because only the full-length Sp3 but not the truncated Sp3 suppresses MnSOD promoter activity. These findings suggest a novel mechanism by which Tat modulates the repression of the MnSOD gene and establish a link between HIV infection and liver cancer.

Transcriptional Regulation of the HumanCD3γ Gene: The TATA-LessCD3γ Promoter Functions via an Initiator and Contiguous Sp-Binding Elements

Growing evidence that the CD3gamma gene is specifically targeted in some T cell diseases focused our attention on the need to identify and characterize the elusive elements involved in CD3gamma transcriptional control. In this study, we show that while the human CD3gamma and CD3delta genes are oriented head-to-head and separated by only 1.6 kb, the CD3gamma gene is transcribed from an independent but weak, lymphoid-specific TATA-less proximal promoter. Using RNA ligase-mediated rapid amplification of cDNA ends, we demonstrate that a cluster of transcription initiation sites is present in the vicinity of the primary core promoter, and the major start site is situated in a classical initiator sequence. A GT box immediately upstream of the initiator binds Sp family proteins and the general transcription machinery, with the activity of these adjacent elements enhanced by the presence of a second GC box 10 nt further upstream. The primary core promoter is limited to a sequence that extends upstream to -15 and contains the initiator and GT box. An identical GT box located approximately 50 nt from the initiator functions as a weak secondary core promoter and likely generates transcripts originating upstream from the +1. Finally, we show that two previously identified NFAT motifs in the proximal promoter positively (NFATgamma(1)) or negatively (NFATgamma(1) and NFATgamma(2)) regulate expression of the human CD3gamma gene by their differential binding of NFATc1 plus NF-kappaB p50 or NFATc2 containing complexes, respectively. These data elucidate some of the mechanisms controlling expression of the CD3gamma gene as a step toward furthering our understanding of how its transcription is targeted in human disease.

Genomic organization and promoter analysis of the mouse ADP-ribosylarginine hydrolase gene

Mono-ADP-ribosylation is a reversible modification of proteins with NAD:arginine ADP-ribosyltransferases and ADP-ribosylarginine hydrolases (ADPRH) catalyzing the opposing arms of an ADP-ribosylation cycle. The ADPRH cDNA had been cloned from human, rat, and mouse tissues and high levels of mRNA were found in brain, spleen, and testis. To begin to understand the molecular mechanisms that regulate ADPRH gene expression, we cloned the full-length cDNA, determined the genomic structure of mouse ADPRH, and investigated promoter function. Northern analysis using different regions of the ADPRH cDNA as probes identified two mRNAs of 1.7 and 3.0 kb, which resulted from the use of alternative polyadenylation signals, CATAAC and ATTAAA, beginning at positions 1501 and 2885, respectively, of the nucleotide sequence (A of ATG = 1). The ADPRH gene, represented in two overlapping genomic clones, spans approximately 9 kilobases with four exons and three introns. The 5'-flanking region contains the features of a housekeeping gene; it has neither a TATA nor a CAAT box, but is, instead, highly GC-rich with multiple transcription initiation sites. Promoter analysis, assessed using transient transfection of PC12, NB41A3, NIH/3T3, and Hepa 1-6 cells with truncated constructs, revealed potent stimulatory (-119 to -89) and inhibitory (-161 to -119) elements, which were utilized similarly in the different cell lines. Further mutational analysis of the promoter and electrophoretic mobility-shift assays identified a positive GC-box element (-107 to -95); Sp1 and Sp3, which bound to this motif, were also detected by supershift assays. In co-transfection experiments using Drosophila SL2 cells that lack endogenous Sp1, Sp1 trans-activated the ADPRH promoter in a manner dependent on the presence of an Sp1-binding motif. The promoter activity pattern and involvement of Sp transcription factors are consistent with prior observations of widespread hydrolase expression in mammalian tissues.

Involvement of specific proteins (Sp1/Sp3) and nuclear factor Y in basal transcription of the distal promoter of the rat pyruvate carboxylase gene in β-cells

Pyruvate carboxylase plays diverse roles in different biosynthetic pathways, including glucose-induced insulin secretion in pancreatic beta-cells. We have localized the control region of the P2 promoter by generating a series of 5'-nested deletion constructs, and both 25- and 9-bp internal deletion constructs, as well as by performing site-directed mutagenesis. Transient transfections of these constructs into INS-1 cells identified a CCAAT box and a GC box that are located at -65/-61 and -48/-41, respectively, as the important determinants. Disruption of the GC box resulted in a 4-fold reduction of the reporter activity, while disruption of the proximal CCAAT box (-65/-61) but not the distal CCAAT box (-95/-91) increased the reporter activity by 3-fold. Simultaneous disruptions of both the GC box and the CCAAT box reduced the reporter activity to a level that was close to that of the single GC box mutation. Electrophoretic mobility shift assays (EMSAs) and supershift EMSAs using nuclear extract from INS-1 cells demonstrated that Sp1 and Sp3 bind a GC box while the nuclear factor Y was shown to bind the proximal but not the distal CCAAT box.

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

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

Specific sequence configurations of HIV-1 LTR G/C box array result in altered recruitment of Sp isoforms and correlate with disease progression

Basal and activated human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) activity, and in return, viral replication is partly dependent on interactions of the G/C box array with the Sp family of transcription factors. Analysis of LTR Sp binding site sequence variants revealed a C-to-T change at position 5 within Sp site III that increased in frequency and a 5T mutation within Sp site II, which decreased in frequency during the course of HIV disease. These results suggest LTR Sp binding site sequence variants may prove useful as viral molecular markers indicative of progressive HIV-1-induced disease.

TRAIL-R as a negative regulator of innate immune cell responses

TRAIL receptor (TRAIL-R) signaling has been implicated in inducing apoptosis in tumor cells, but little is understood about its physiological function. Here, we report the generation and characterization of TRAIL-R(-/-) mice, which develop normal lymphocyte populations but possess enhanced innate immune responses. TRAIL-R(-/-) mice exhibited increased clearance of murine cytomegalovirus that correlated with increased levels of IL-12, IFN-alpha, and IFN-gamma. Stimulation of macrophages with Mycobacterium and Toll-like receptor (TLR)-2, -3, and -4, but not TLR9, ligands resulted in high levels of TRAIL upregulation and enhanced cytokine production in TRAIL-R(-/-) cells. The immediate-early TLR signaling events in TRAIL-R(-/-) macrophages and dendritic cells are normal, but I kappa B-alpha homeostatic regulation and NF-kappa B activity at later time points is perturbed. These data suggest that TRAIL-R negatively regulates innate immune responses.

Human transcription factor Sp3: genomic structure, identification of a processed pseudogene, and transcript analysis

The human transcription factor Sp3 has been widely studied at the translational level and has been described as a regulatory factor for a number of genes. Sp3 is currently characterized as a bifunctional transcription factor having the ability to behave as both an activator and/or a repressor in various promoter regions. Previous translational studies have attempted to determine the basis for these diverse functions with mostly contradictory evidence to date. Little data are available, however, concerning genomic structure, full-length cDNA, potential transcript variants, or location of translation initiation sites for the large isoform of the Sp3 gene. In this study, bacterial artificial chromosome (BAC) sequencing, reverse transcription-polymerase chain reaction (RT-PCR), genomic PCR, and database mining indicate that the Sp3 gene encompasses seven exons spanning more than 55 kb of genomic DNA on Chromosome 2. The 5' end of this sequence contains a large CpG island. This work also detected a processed pseudogene, psiSp3, located on Chromosome 13, spanning approximately 4.0 kb. Northern blot analysis detected three predominant transcripts at 4.0, 6.0 and 2.5 kb. Sequence analysis indicated that alternative splicing of exon 3 allows for multiple transcripts of Sp3. Each sequenced transcript possesses three to five potential translation initiation sites. This diversity at the level of gene expression will likely be key to understanding the diverse functions of Sp3.

A General Survey of Thymocyte Differentiation by Transcriptional Analysis of Knockout Mouse Models

The thymus is the primary site of T cell lymphopoiesis. To undergo proper differentiation, developing T cells follow a well-ordered genetic program that strictly depends on the heterogeneous and highly specialized thymic microenvironment. In this study, we used microarray technology to extensively describe transcriptional events regulating alphabeta T cell fate. To get an integrated view of these processes, both whole thymi from genetically engineered mice together with purified thymocytes were analyzed. Using mice exhibiting various transcriptional perturbations and developmental blockades, we performed a transcriptional microdissection of the organ. Multiple signatures covering both cortical and medullary stroma as well as various thymocyte maturation intermediates were clearly defined. Beyond the definition of histological and functional signatures (proliferation, rearrangement), we provide the first evidence that such an approach may also highlight the complex cross-talk events that occur between maturing T cells and stroma. Our data constitute a useful integrated resource describing the main gene networks set up during thymocyte development and a first step toward a more systematic transcriptional analysis of genetically modified mice.

Sp1 and Sp3 transcription factors mediate trichostatin A-induced and basal expression of extracellular superoxide dismutase

Extracellular superoxide dismutase (EC-SOD) is the major extracellular antioxidant enzyme and may play a critical role in the pathogenesis of a variety of pulmonary, neurological, and cardiovascular diseases. We report here that exposure to the deacetylase inhibitor trichostatin A (TSA) induces EC-SOD mRNA levels in mIMCD3 and Hepa 1-6 cells, but reduces EC-SOD mRNA levels in MLg cells. To determine the molecular mechanism of TSA-mediated EC-SOD gene regulation, we analyzed EC-SOD's proximal promoter region, which revealed two previously unknown but putative Sp1 cis elements. Transfection of systematically truncated 5'-flanking sequences revealed that the second Sp1 binding site contributes up to 70% of the constitutive EC-SOD promoter activity. Binding of Sp1 and Sp3 transcription factors to this region was confirmed by DNase I footprinting, electrophoretic mobility shift assay, super-shift assay, and chromatin immunoprecipitation. A dominant-negative Sp1 construct considerably reduced EC-SOD promoter activity in mammalian cells, whereas coexpression of Sp1 and Sp3 greatly enhanced reporter activity in SL2 cells. An EC-SOD promoter-reporter construct showed from 5- to 14-fold induction after exposure to TSA, whereas deletion of the Sp1 binding site significantly reduced reporter activation. These results are consistent with Sp1/Sp3 transcription factors providing essential TSA-dependent and basal transcription of the EC-SOD gene and may represent a novel pharmacological pathway for regulating EC-SOD levels in tissue.

Sp3 repression of polymorphic human NRH:quinone oxidoreductase 2 gene promoter

Human NRH:quinone oxidoreductase 2 (NQO2) gene-containing 29-bp deletion/insertion polymorphic promoters were found to be associated with susceptibility to Parkinson's disease. Here, we demonstrate that the NQO2 gene is differentially expressed by the polymorphic promoters in human fibroblasts and Hep-G2 cells transfected with NQO2 gene reporter constructs. Promoter containing the 29-bp insertion polymorphism demonstrated significantly lower NQO2 gene expression. Deletion mutagenesis and DNase I footprinting analysis of the promoter without the 29-bp insertion identified three protected regions (region A, B, and C). Band- and supershift and transfection assays showed binding of transcription factor Sp1 to regions A and B, which regulated expression of the NQO2 gene. Similar studies of the NQO2 gene promoter with the 29-bp insertion polymorphism showed that regions A and C were identical and contributed similarly as in the promoter without the 29-bp insertion to NQO2 gene expression. However, region B was found to be inserted with 29-bp DNA element and bound to both Sp1 and Sp3. Binding of Sp3 led to repression of NQO2 gene transcription by the promoter containing the 29-bp insertion polymorphism. These studies also suggest that alterations in NQO2 activity might be an important factor in susceptibility to Parkinson's disease.

Zinc finger transcription factors in skeletal development

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

Transcriptional Regulation of the Human β-1,4-Galactosyltransferase V Gene in Cancer Cells

Beta-1,4-galactosyltransferase (beta-1,4-GalT) V is a constitutively expressed enzyme that can effectively galactosylate the GlcNAcbeta1-->6Man group of the highly branched N-glycans that are characteristic of tumor cells. Upon malignant transformation of cells, the expression of the beta-1,4-GalT V gene increases in accordance with the increase in the amounts of highly branched N-glycans. Lectin blot analysis showed that the galactosylation of highly branched N-glycans is inhibited significantly in SH-SY5Y human neuroblastoma cells by the transfection of the antisense beta-1,4-GalT V cDNA, indicating the biological importance of the beta-1,4-GalT V for the functions of highly branched N-glycans. We cloned the 2.3-kb 5'-flanking region of the human beta-1,4-GalT V gene, and we identified the region -116/-18 relative to the transcription start site as that having promoter activity. The region was found to contain several putative binding sites for transcription factors, including AP2, AP4, N-Myc, Sp1, and upstream stimulatory factor. Electrophoretic mobility shift assay showed that Sp1 binds to nucleotide positions -81/-69 of the promoter region. Mutations induced in the Sp1-binding site showed that the promoter activity of the beta-1,4-GalT V gene is impaired completely in cancer cells. In contrast, the promoter activity increased significantly by the transfection of the Sp1 cDNA into A549 human lung carcinoma cells. Mithramycin A, which inhibits the binding of Sp1 to its binding site, reduced the promoter activation and expression of the beta-1,4-GalT V gene in A549 cells. These results indicate that Sp1 plays an essential role in the transcriptional activity of the beta-1,4-GalT V gene in cancer cells.

Gene regulation by Sp1 and Sp3

The Sp family of transcription factors is united by a particular combination of three conserved Cys2His2 zinc fingers that form the sequence-specific DNA-binding domain. Within the Sp family of transcription factors, Sp1 and Sp3 are ubiquitously expressed in mammalian cells. They can bind and act through GC boxes to regulate gene expression of multiple target genes. Although Sp1 and Sp3 have similar structures and high homology in their DNA binding domains, in vitro and in vivo studies reveal that these transcription factors have strikingly different functions. Sp1 and Sp3 are able to enhance or repress promoter activity. Regulation of the transcriptional activity of Sp1 and Sp3 occurs largely at the post-translational level. In this review, we focus on the roles of Sp1 and Sp3 in the regulation of gene expression.Key words: Sp1, Sp3, gene regulation, sub-cellular localization.

Complexity of translationally controlled transcription factor Sp3 isoform expression

Sp3 is a ubiquitous transcription factor closely related to Sp1. Both proteins contain a highly conserved DNA-binding domain close to the C terminus and two glutamine-rich domains in the N-terminal moiety. Immunoblot analyses of Sp3 reveal a striking complex protein pattern of up to eight distinct species. This pattern is not observed in Sp3-deficient cell lines showing that all signals reflect Sp3 antigen. In this study, we have unraveled the complexity of Sp3 expression. We show that four isoforms of Sp3 that retain different parts of the N terminus are expressed in vivo. The four isoforms derive from alternative translational start sites at positions 1, 37, 856, and 907. An upstream open reading frame located at position -47 to -18 regulates expression of the two long isoforms. Unlike Sp1, none of the Sp3 isoforms is glycosylated. However, all four isoforms become SUMO-modified in vivo and in vitro specifically and exclusively at lysine residue 551. The transcriptional activity of the two long isoforms strongly depends on the promoter settings, whereas the small isoforms appear to be inactive. The transcriptional activity of all the Sp3 isoforms is regulated by SUMO modification. Our results demonstrate that Sp3 has many unique features and is not simply a functional equivalent of Sp1.

Methylation in the Core-promoter Region of the Chondromodulin-I Gene Determines the Cell-specific Expression by Regulating the Binding of Transcriptional Activator Sp3

Transcriptional regulation of cell- and stage-specific genes is a crucial process in the development of mesenchymal tissues. Here we have investigated the regulatory mechanism of the expression of the chondromodulin-I (ChM-I) gene, one of the chondrocyte-specific genes, in osteogenic cells using osteosarcoma (OS) cells as a model. Methylation-specific sequence analyses revealed that the extent of methylation in the core-promoter region of the ChM-I gene was correlated inversely with the expression of the ChM-I gene in OS primary tumors and cell lines. 5-Aza-deoxycytidine treatment induced the expression of the ChM-I gene in ChM-I-negative OS cell lines, and the induction of expression was associated tightly with the demethylation of cytosine at -52 (C(-52)) in the middle of an Sp1/3 binding site to which the Sp3, but not Sp1, bound. The replacement of C(-52) with methyl-cytosine or thymine abrogated Sp3 binding and also the transcription activity of the genomic fragment including C(-52). The inhibition of Sp3 expression by small interfering RNA reduced the expression of the ChM-I gene in ChM-I-positive normal chondrocytes, indicating Sp3 as a physiological transcriptional activator of the ChM-I gene. These results suggest that the methylation status of the core-promoter region is one of the mechanisms to determine the cell-specific expression of the ChM-I gene through the regulation of the binding of Sp3.

Molecular Cloning and Functional Characterization of the Transcription Factor YY2

YY1 is a ubiquitous zinc finger transcription factor that binds to and regulates promoters and enhancers of many cellular and viral genes. Here we report the isolation of a human cDNA encoding a DNA sequence-specific binding protein with significant homology to the transcription factor YY1. A sequence analysis of this novel protein, YY2, revealed an overall 65% identity in the DNA sequence and a 56% identity in protein sequence compared with human YY1. The most pronounced similarity between YY1 and YY2 exists within the zinc finger regions of the two proteins, and consistent with this observation, YY2 can bind to and regulate some promoters known to be controlled by YY1. Similar to YY1, YY2 contains both transcriptional activation and repression functions. The finding of a protein with structure and function similar to YY1 provides a new opportunity to explore additional mechanisms by which YY1-responsive genes can be regulated and suggests that gene regulation by YY1 is far more complicated than previously assumed.

Sp2 DNA Binding Activity and trans-Activation Are Negatively Regulated in Mammalian Cells

Previous studies have indicated that Sp2 binds poorly to GC-rich sequences bound by Sp1 and Sp3, and further functional analyses of Sp2 have been limited. To study Sp2-mediated transcription, we employed a PCR-based protocol to determine the Sp2 consensus DNA-binding sequence (5'-GGGCGGGAC-3') and performed kinetic experiments to show that Sp2 binds this consensus sequence with high affinity (225 pm) in vitro. To determine the functional consequence of Sp2 interaction with this sequence in vivo, we transformed well characterized Sp-binding sites within the dihydrofolate reductase (DHFR) promoter to consensus Sp2-binding sites. Incorporation of Sp2-binding sites within the DHFR promoter increased Sp2-mediated trans-activation in transient co-transfection experiments but also revealed Sp2 to be a relatively weak trans-activator with little or no capacity for additive or synergistic trans-activation. Using chimeric molecules prepared with portions of Sp1 and Sp2 and the human prostate-specific antigen promoter, we show that Sp2 DNA binding activity and trans-activation are negatively regulated in mammalian cells. Taken together, our data indicate that Sp2 is functionally distinct relative to other Sp family members and suggest that Sp2 may play a unique role in cell physiology.

SP3 acts as a positive regulator on the core promoter of human ZPK gene

ZPK (zipper protein kinase)/MUK/DLK/MAP3K12, a member of mixed-lineage kinases (MLKs), is expressed in a tissue-specific manner, particularly in developing brain, and likely to contribute to cytodifferentiation, apoptotic elimination, and migration. To understand the preferential expression of ZPK in neuronal tissues, we have analyzed the putative core promoter region upstream of the first exon of the human ZPK gene. The core promoter region is TATA-less, but contains several potential transcription factor-binding motifs such as a GC-box, all of which are well conserved between human and mouse. Reporter assays and 'gel-shift' analysis using SH-SY5Y cells revealed that a xenobiotic responsive element (XRE)-like motif (GGGCGTGTCC) was preferentially recognized by Sp3, and enhanced the core promoter activity. However, the core promoter activity was still potent even in HeLa cells which barely express ZPK. Our results suggest that, for the selective expression of ZPK gene, cell-specific negative regulatory element(s) which locate outside of the core promoter region repress the potent basic promoter activity.

Regulation of RNA Polymerase II Transcription by Sequence-Specific DNA Binding Factors

In eukaryotes, transcription of the diverse array of tens of thousands of protein-coding genes is carried out by RNA polymerase II. The control of this process is predominantly mediated by a network of thousands of sequence-specific DNA binding transcription factors that interpret the genetic regulatory information, such as in transcriptional enhancers and promoters, and transmit the appropriate response to the RNA polymerase II transcriptional machinery. This review will describe some early advances in the discovery and characterization of the sequence-specific DNA binding transcription factors as well as some of the properties of these regulatory proteins.

mRNA 5′ region sequence incompleteness: a potential source of systematic errors in translation initiation codon assignment in human mRNAs

The amino acid sequence of gene products is routinely deduced from the nucleotide sequence of the relative cloned cDNA, according to the rules for recognition of start codon (first-AUG rule, optimal sequence context) and the genetic code. From this prediction stem most subsequent types of product analysis, although all standard methods for cDNA cloning are affected by a potential inability to effectively clone the 5' region of mRNA. Revision by bioinformatics and cloning methods of 109 known genes located on human chromosome 21 (HC 21) shows that 60 mRNAs lack any in-frame stop upstream of the first-AUG, and that in five cases (DSCR1, KIAA0184, KIAA0539, SON, and TFF3) the coding region at the 5' end was incompletely characterized in the original descriptions. We describe the respective consequences for genomic annotation, domain and ortholog identification, and functional experiments design. We have also analyzed the sequences of 13,124 human mRNAs (RefSeq databank), discovering that in 6448 cases (49%), an in-frame stop codon is present upstream of the initiation codon, while in the other 6676 mRNAs (51%), identification of additional bases at the mRNA 5' region could well reveal some new upstream in-frame AUG codons in the optimal context. Proportionally to the HC 21 data, about 550 known human genes might thus be affected by this 5' end mRNA artifact.

Sp1 and Sp3 Are important regulators of AP-2gamma gene transcription

AP-2gamma is a member of the AP-2 transcription factor family, is highly enriched in the trophoblast cell lineage, and is essential for placental development. In an effort to identify factors regulating AP-2gamma gene expression, we isolated and characterized the promoter and 5'-flanking region of the mouse and human AP-2gamma genes. The transcription start site of the mouse AP-2gamma gene was mapped by primer extension and 5' rapid amplification of cDNA ends. Deletion analysis of the 5'-flanking region revealed a 704-base pair (bp) sequence located approximately 6 kilobases (kb) upstream of the transcription start site that is required for enhanced expression in trophoblast cells. Additional gene transfer studies showed that basal promoter activity resides within a highly conserved, approximately 200-bp DNA sequence located immediately upstream of the transcription start site. The conserved region is highly GC-rich and lacks typical TATA or CCAAT boxes. Multiple potential Sp- and AP-2-binding sites are clustered within this region. Electrophoretic mobility shift assays demonstrated that Sp1 and Sp3 bind to three sites in the promoter region of the mouse AP-2gamma gene. Combined mutation of the three putative Sp sites reduced promoter activity by 80% in trophoblast and nontrophoblast cells, demonstrating the functional importance of these sites in regulating AP-2gamma gene expression. In summary, we have identified a potential trophoblast cell-specific regulatory element located approximately 6 kb upstream of the murine AP-2gamma gene transcription start site, and we have shown that Sp1 and Sp3 bind to cis-regulatory elements located in the promoter proximal region and contribute to basal promoter activity.

The PKR kinase promoter binds both Sp1 and Sp3, but only Sp3 functions as part of the interferon-inducible complex with ISGF-3 proteins

The protein kinase regulated by RNA (PKR) is an important mediator of the antiviral and antiproliferative actions of interferon (IFN). The promoter of the PKR gene contains a novel 15-bp element designated KCS that is required for both basal and IFN-inducible transcription, with KCS function dependent upon both position and orientation relative to the ISRE element. Novel inducible protein complexes (iKIBP1, iKIBP2) that require both the KCS and the ISRE element sequences for their formation have been identified and characterized. Transcription factors Sp1 and Sp3 were found to be KCS-binding proteins by electrophoretic mobility shift analyses (EMSA) and Sepharose bead-KCS oligonucleotide pull-down assays. However, only Sp3 but not Sp1 was a constituent of the inducible iKIBP complexes. EMSA also identified STAT1, STAT2, and IRF-9 as components of the iKIBP complexes, indicating that ISGF-3 participates in iKIBP complex formation. Proteins bound at the KCS element in the absence of ISRE were able to recruit both STAT1 and STAT2 to the KCS element; recruitment was dependent upon IFN-alpha treatment. Chromatin immunoprecipitation assays revealed that the binding of Sp3, similar to STAT1 and STAT2, at the PKR promoter in vivo was IFN-dependent, but that Sp1 binding was not dependent upon IFN treatment. These results, taken together, strongly suggest a role for Sp1 in basal and Sp3 in inducible transcription of PKR and that a potential function of the KCS element is to facilitate the recruitment of ISGF-3 complex components to the PKR promoter to stimulate transcription.

N-myc oncogene expression in neuroblastoma is driven by Sp1 and Sp3

Regulation of N-myc oncogene expression is an important determinant of the biological behavior of neuroblastoma. The N-myc promoter contains several potential binding sites for transcription factors of the Sp1 family. Mutation of a CT-box motif contained within a 26 bp region required for N-myc downregulation by retinoic acid decreased basal transcriptional activity and altered DNA-protein interactions of the promoter, while mutations flanking this motif did neither. On super-shift, this region was shown to recruit Sp1 and Sp3 transcription factor proteins, while a functionally significant CT-box mutation resulted in their replacement by NF-1 transcription factor. Lysates from Drosophila S2 cells expressing exogenous Sp1, Sp3, and NF-1 proteins were able to partially mimic gel shift complexes seen with neuroblastoma nuclear extract and either wild type or mutant probes. Transient transfections of S2 cells showed that both individually and together, Sp1 and Sp3 were able to trans-activate a wild type CT-box-driven luciferase reporter construct in a dose-dependent manner. Transfection of the wild type but not mutant CT-box oligonucleotide was able to decrease endogenous N-myc expression in neuroblastoma cells. Together these results suggest that the CT-box element serves a critically functional role, and in the basal state, allows for N-myc trans-activation by Sp1 and Sp3. Moreover when mutated, the CT-box may still function as a binding motif for alternate transcription factors such as NF-1 that can allow persistent N-myc expression.

Functional characterization of transcription factor binding sites for HNF1-alpha, HNF3-beta (FOXA2), HNF4-alpha, Sp1 and Sp3 in the human prothrombin gene enhancer

BACKGROUND

Prothrombin is a key component in blood coagulation. Overexpression of prothrombin leads to an increased risk of venous thrombosis. Therefore, the study of the transcriptional regulation of the prothrombin gene may help to identify mechanisms of overexpression.

OBJECTIVES

The aim of our study was to localize the regions within the prothrombin enhancer responsible for its activity, to identify the proteins binding to these regions, and to establish their functional importance.

METHODS

We constructed a set of prothrombin promoter 5' deletion constructs containing the firefly luciferase reporter gene, which were transiently transfected in HepG2, HuH7 and HeLa cells. Putative transcription factor (TF) binding sites were evaluated by electrophoretic mobility shift assays. The functional importance of each TF binding site was evaluated by site directed mutagenesis and transient transfection of the mutant constructs.

RESULTS

We confirmed the major contribution of the enhancer region to the transcriptional activity of the prothrombin promoter. Analysis of this region revealed putative binding sites for hepatocyte nuclear factor HNF4, HNF3-beta and specificity protein(Sp)1. We identified six different TFs binding to three evolutionary conserved sites in the enhancer: HNF4-alpha (site 1), HNF1-alpha, HNF3-beta and an as yet unidentified TF (site 2) and the ubiquitously expressed TFs Sp1 and Sp3 (site 3). Mutagenesis studies showed that loss of binding of HNF3-beta resulted in a considerable decrease of enhancer activity, whereas loss of HNF4-alpha or Sp1/Sp3 resulted in milder reductions.

CONCLUSIONS

The prothrombin enhancer plays a major role in regulation of prothrombin expression. Six different TFs are able to bind to this region. At least three of these TFs, HNF4-alpha, HNF3-beta and Sp1/Sp3, are important in regulation of prothrombin expression.

Impaired hematopoiesis in mice lacking the transcription factor Sp3

As the zinc-finger transcription factor specificity protein 3 (Sp3) has been implicated in the regulation of many hematopoietic-specific genes, we analyzed the role of Sp3 in hematopoiesis. At embryonic day 18.5 (E18.5), Sp3-/- mice exhibit a partial arrest of T-cell development in the thymus and B-cell numbers are reduced in liver and spleen. However, pre-B-cell proliferation and differentiation into immunoglobulin M-positive (IgM+) B cells in vitro are not affected. At E14.5 and E16.5, Sp3-/- mice exhibit a significant delay in the appearance of definitive erythrocytes in the blood, paralleled by a defect in the progression of differentiation of definitive erythroid cells in vitro. Perinatal death of the null mutants precludes the analysis of adult hematopoiesis in Sp3-/- mice. We therefore investigated the ability of E12.5 Sp3-/- liver cells to contribute to the hematopoietic compartment in an in vivo transplantation assay. Sp3-/- cells were able to repopulate the B- and T-lymphoid compartment, albeit with reduced efficiency. In contrast, Sp3-/- cells showed no significant engraftment in the erythroid and myeloid lineages. Thus, the absence of Sp3 results in cell-autonomous hematopoietic defects, affecting in particular the erythroid and myeloid cell lineages.

Regulation of human CETP gene expression: role of SP1 and SP3 transcription factors at promoter sites -690, -629, and -37

Cholesteryl ester transfer protein (CETP) is a key factor in plasma reverse cholesterol transport and is implicated in the pathophysiology of atherogenic dyslipidemia. Variations observed in plasma CETP mass and activity in both normolipidemic and dyslipidemic individuals may reflect differences in CETP gene expression. We evaluated the respective roles of the Sp1 and Sp3 transcription factors on the promoter activity of the human CETP gene at a new Sp1/Sp3 site identified at position -690, and at two previously described Sp1/Sp3 sites at positions -37 and -629. In transient transfection in HepG2 cells, site-directed mutagenesis using luciferase reporter constructs containing a promoter fragment from +32 to -745 indicated that the new -690 site acts as a repressive element in reducing CETP promoter activity (-22%; P < 0.05); equally, this site exerts an additive effect with the -629 site, inducing marked repression (-42%; P < 0.005). In contrast, in NCTC cells that display a 16-fold lower level of Sp3, the repressive effect at the -690 site was enhanced 2-fold (-45%; P < 0.05), whereas the -629 site exerted no effect. Cotransfection of Sp1 and/or Sp3 in SL2 insect cells lacking endogenous Sp factors demonstrated that Sp1 and Sp3 act as activators at the -690 and -37 sites, whereas Sp3 acts as a repressor at the -629 site. Taken together, our data demonstrate that Sp1 and Sp3 regulate human CETP promoter activity through three Sp1/Sp3 binding sites in a distinct manner, and that the Sp1/Sp3 ratio is a key factor in determining the relative contribution of these sites to total promoter activity.

ACTH modulation of transcription factors responsible for steroid hydroxylase gene expression in the adrenal cortex

Steroid hormone biosynthesis in the adrenal cortex and gonads involves the coordinated transcription of the genes encoding the steroid hydroxylases, 3beta-hydroxysteroid dehydrogenase (3betaHSD), the steroidogenic acute regulatory protein (StAR), and adrenodoxin (Adx). Transcriptional regulation of steroidogenic genes is multifactorial, entailing developmental, tissue-specific, constitutive, and cAMP-dependent mechanisms. Optimal steroidogenic capacity is achieved by the actions of ACTH which exerts transcriptional pressure on all steroidogenic genes. The actions of ACTH in the adrenal cortex have been studied in great detail and is mediated by cAMP and protein kinase A (PKA) via two temporally distinct pathways. The acute response leads to mobilization of cholesterol, the initial substrate for all steroidogenic pathways, from cellular stores to the inner mitochondrial membrane where cholesterol sidechain cleavage cytochrome P450 (P45011A1) resides. The slower, chronic response of ACTH in the adrenal cortex directs transcription of the genes encoding the steroidogenic enzymes. Although steroidogenic gene transcription in response to ACTH is cAMP-dependent, the consensus cAMP response pathway (CRE/CREB) is not involved. Instead, each steroidogenic gene utilizes unique cAMP-responsive sequences (CRS) found in the promoters of each gene, which bind a diverse array of transcription factors. Moreover, once specific transcription factors are bound to the promoters of the steroidogenic genes, increased gene expression requires posttranslational modification (phosphorylation/dephosphorylation) of the transcription factors and binding of coactivator proteins. This review provides a general view (with emphasis on the human) of the important factors involved in regulating steroidogenic gene expression and ultimately steroid hormone biosynthesis.

Transcriptional control of the RECK metastasis/angiogenesis suppressor gene

The RECK gene is widely expressed in normal human tissues but is downregulated in tumor cell lines and oncogenically transformed fibroblasts. RECK encodes a membrane-anchored glycoprotein that suppresses tumor invasion and angiogenesis by regulating matrix-metalloproteinases (MMP-2, MMP-9 and MT1-MMP). Understanding of the transcriptional regulation of tumor/metastasis suppressor genes constitutes a potent approach to the molecular basis of malignant transformation. In order to uncover the mechanisms of control of RECK gene expression, the RECK promoter has been cloned and characterized. One of the elements responsible for the Ras-mediated downregulation of mouse RECK gene is the Sp1 site, to which Sp1 and Sp3 factors bind. Other regulatory events, such as DNA methylation of the RECK promoter and histone acetylation/deacetylation have been studied to understand the underlying mechanisms of RECK expression. Understanding of the mechanisms which control RECK gene transcription may lead to the development of new strategies for cancer prevention and treatment.

Sp family of transcription factors is involved in valproic acid-induced expression of Galphai2

Valproic acid-induced gene expression has been attributed to the DNA-binding activity of the transcription factor activator protein 1 (AP-1). Using K562 cells, we have studied valproic acid-induced transcription from the human Galpha(i2) gene promoter, which lacks AP-1-binding motifs. We find that valproic acid-induced expression of Galpha(i2) is inhibited by mithramycin A, a compound that interferes with Sp1 binding to GC boxes in DNA. Three Sp1-binding sequences, located at +68/+75, -50/-36, and -92/-85 in the promoter, accounted for about 60% of this transcriptional effect, as judged by transient transfection assays. Electrophoretic mobility shift assays indicated that these sites bind members of the Sp family of transcription factors. Binding to DNA was inhibited by mithramycin A and was greater in nuclear extracts from cells treated with valproic acid than in control cells. Okadaic acid, calyculin A, and fostriecin, which are potent inhibitors of protein phosphatase, suppressed the transcriptional response to valproic acid. This inhibitory effect was not observed when promoter constructs containing mutations in the referenced Sp1-binding sites were used for transfections. In nuclear extracts from cells cultured in the presence of these inhibitors, the binding of Sp1/Sp3 to DNA probes was much less than in control cells. Alkaline phosphatase treatment of nuclear extracts resulted in enhanced binding of Sp proteins to the DNA probes. These results are consistent with the idea that dephosphorylating conditions enhanced Sp binding to the DNA probes as well as Sp-mediated transcription induced by valproic acid. This study demonstrates that the gene expression-inducing effect of valproic acid occurs, in part, through the Sp family of transcription factors.

A GC-box within the proximal promoter region of the rat cathepsin L gene activates transcription in Sertoli cells of sexually mature rats

It has been proposed that stage-specific gene expression in Sertoli cells results from sequential activation and repression of transcription. However, the exact molecular mechanisms are unknown. As a first step in addressing this fundamental issue, we recently demonstrated that a 3-kilobase (kb) genomic fragment immediately upstream of the rat cathepsin L translation start site directed stage-specific expression of a reporter gene only in Sertoli cells of transgenic mice in a manner comparable to that of the endogenous gene (predominantly in stages VI-VIII tubules). Supporting the activation/repression model of regulation, an upstream domain that mediated an inhibitory effect by male germ cells was identified within this 3-kb promoter region. In the present study, we localized and characterized the regulatory elements that activate transcription. Analyses of a series of 5' deletion constructs demonstrated that a 120-base pair (bp) region that spans the transcription start site of the rat cathepsin L gene was sufficient to activate transcription in Sertoli cells isolated from sexually mature rats. Within this region, electrophoretic mobility shift assays showed that one member of the Sp/XKLF family of factors, Sp3, specifically bound to a GC-box. Furthermore, Sp1-binding activity was not detected in nuclear extracts from Sertoli cells of sexually mature rats. Finally, the GC-box was shown to be essential for promoter activity since mutating this binding motif abolished promoter activity. Collectively, these results suggest that the GC-box is a critical regulatory element for the cathepsin L promoter in mature Sertoli cells.

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

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

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

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

Expression of the human Hand1 gene in trophoblastic cells is transcriptionally regulated by activating and repressing specificity protein (Sp)-elements

The tissue-specific basic helix-loop-helix protein Hand1 is essential for the formation of trophoblast giant cells of the murine placenta. In humans, Hand1 is detectable in trophoblastic tumour cells suggesting an equivalent role in trophoblast differentiation. To understand its mode of expression we have cloned and characterized the human Hand1 gene promoter. Primer extension analyses suggest that transcription initiates 19 nucleotides downstream of the TATA element of the proximal 5' flanking region. Expression of luciferase reporter constructs harboring deletions of the 9.5 kb Hand1 5' flanking sequence defines a promoter region within 274 bp upstream of the transcriptional start site. Compared to a reporter bearing only the TATA box, the proximal promoter activates transcription up to 30-fold. However, transcriptional activity of the region was observed in both Hand1-expressing and non-expressing cell lines. Sequencing, DNAseI footprint analyses and electrophoretic mobility shift assays reveal the presence of four GC-rich sequences, which show different affinities to the endogenous specificity proteins (Sp), and a CCAAT box. In vitro, the Sp-elements mainly interact with Sp1 and Sp3 while the CCAAT element is recognized by the alpha CAAT binding factor protein. Mutant luciferase reporters bearing single active or inactive recognition sites demonstrate that two of the four Sp-binding sites (I and IV) contribute little to the overall transcription rate. The two other Sp-cognate sequences, II and III, downregulate and activate reporter expression 2.3- and 2.6-fold, respectively. Co-transfections of Sp1/Sp3 expression vectors and mutated reporter constructs in Sp-deficient SL2 cells indicate that the Sp-binding site II and III indeed function as repressing and activating enhancer sequences. In summary, the data suggest that constitutive expression of the Hand1 gene in cultured cells is regulated by a complex interplay of Sp-proteins interacting with activator and repressor elements.

Sp1 and Sp3 transcription factors synergistically regulate HGF receptor gene expression in kidney

We investigated the expression pattern and underlying mechanism that controls hepatocyte growth factor (HGF) receptor (c-met) expression in normal kidney and a variety of kidney cells. Immunohistochemical staining showed widespread expression of c-met in mouse kidney, a pattern closely correlated with renal expression of Sp1 and Sp3 transcription factors. In vitro, all types of kidney cells tested expressed different levels of c-met, which was tightly proportional to the cellular abundances of Sp1 and Sp3. Both Sp1 and Sp3 bound to the multiple GC boxes in the promoter region of the c-met gene. Coimmunoprecipitation suggested a physical interaction between Sp1 and Sp3. Functionally, Sp1 markedly stimulated c-met promoter activity. Although Sp3 only weakly activated the c-met promoter, its combination with Sp1 synergistically stimulated c-met transcription. Conversely, deprivation of Sp proteins by transfection of decoy Sp1 oligonucleotide or blockade of Sp1 binding with mithramycin A inhibited c-met expression. The c-met receptor in all types of kidney cells was functional and induced protein kinase B/Akt phosphorylation in a distinctly dynamic pattern after HGF stimulation. These results indicate that members of the Sp family of transcription factors play an important role in regulating constitutive expression of the c-met gene in all types of renal cells. Our findings suggest that HGF may have a broader spectrum of target cells and possess wider implications in kidney structure and function than originally thought.

Transcriptional regulation of the epiregulin gene in the rat ovary

Ovarian follicular development is initiated by FSH secreted from the pituitary gland. The FSH-induced follicular development involves granulosa cell proliferation and differentiation. We demonstrated that a growth factor of epidermal growth factor (EGF) family epiregulin was rapidly induced in the primary culture of rat ovarian granulosa cells by FSH within 1 h. Epiregulin gene expression was also observed in granulosa cells of antral ovarian follicles from pregnant mare's serum gonadotropin-primed rats in vivo. To analyze the regulation of gene expression of epiregulin, we isolated and characterized the rat epiregulin gene of 22.1 kb, including 3.8 kb of 5'-upstream region as well as all five exons and four introns. We determined the transcriptional start site of rat epiregulin gene by primer extension analysis and then characterized the upstream promoter region of the gene. By using a luciferase reporter system, deletion and mutation analyses of rat epiregulin gene promoter region revealed that 125 bp upstream of transcriptional start site was essential, and that two CT boxes and one GT box within this region were important for the gene expression. We also demonstrated by EMSAs that Sp1/Sp3 proteins were involved in the epiregulin gene expression via the upstream sequence. Involvement of Sp1/Sp3 was also demonstrated that transfection of Sp1 or Sp3 expression plasmids dramatically increased the epiregulin gene promoter activities about 90- or 7.9-fold, respectively, in Drosophila SL2 cells that lack endogenous Sp family proteins. Such an increase in the promoter activity was also observed in mammalian cells when NIH-3T3 cells were used. In conclusion, we demonstrated here for the first time that EGF-type growth factor epiregulin is rapidly and strongly induced in the ovarian granulosa cells by FSH stimulation, and that two CT boxes and one GT box present in the upstream region are essential for the promoter activity of rat epiregulin. We also demonstrated that Sp family members play crucial roles in the epiregulin promoter activity through the CT boxes. The restricted and hormonally regulated expression of epiregulin in the rat ovarian granulosa cells may correspond to the physiological relevance of this peptide growth factor to the FSH-induced ovarian follicular growth and maturation.

Binding of Sp1 to the proximal promoter links constitutive expression of the human uPA gene and invasive potential of PC3 cells

Activated transcription of the urokinase-type plasminogen activator (uPA) gene depends on the enhancer, located approximately 2 kb from the start of transcription. The proximal promoter, driving basal transcription, contains a GC-/GA-rich sequence immediately upstream of the TATA box. We have investigated the role played by this element in the transcription of the uPA gene in HeLa and PC3 cells, which do not express or constitutively express the gene, respectively. This region binds either Sp1 or Sp3, as monomers or multimers, but not a combination of the 2 proteins. The more efficient binding of Sp1 to the proximal promoter in PC3 cells is correlated to its phosphorylation state. Polymerase chain reaction (PCR)-coupled, chromatin immunoprecipitation experiments with anti-Sp1 antibodies indeed show an enrichment of proximal promoter sequences in PC3 cells and support the observed difference in transcription levels from proximal promoter constructs in HeLa versus PC3 cells. Furthermore, overexpression of Sp1 increases transcription from the reporter construct in HeLa cells, whereas in PC3 cells, overexpression of Sp3 does not reduce transcription from the same construct, indicating that the Sp1/Sp3 balance cannot be shifted. We conclude that the GC-/GA-rich element of the uPA regulatory region is an independent functional element, regulated by Sp family proteins. Phosphorylation of Sp1 determines the presence in vivo and the functionality of this element in PC3 cells. Thus, the cellular context determines the relevance of the GC-/GA-rich region in uPA gene transcription, which contributes to constitutive gene expression, related, in turn, to the invasive phenotype.

Induction of the mouse kappa-opioid receptor gene by retinoic acid in P19 cells

The mouse kappa-opioid receptor (KOR) gene is expressed in mouse embryonal carcinoma P19 cells and induced by retinoic acid (RA) within 24 h. An RA-responsive cis-acting element is identified within promoter I of the KOR gene. This element contains a GC box, a putative binding site for transcription factor Sp1. Enhanced binding of Sp1 to this GC box correlates with RA induction of KOR gene. Phosphatase inhibitor (sodium pyrophosphate) decreases RA induction of this promoter, whereas hypophosphorylation of Sp1 results in an increase in its DNA binding affinity to this promoter as demonstrated by in vitro gel retardation and in vivo chromatin immunoprecipitation assays. Consistently, the inhibitor of MEK, PD98058, dose-dependently enhances RA induction of this promoter, suggesting that the ERK signaling pathway is negatively involved in the RA induction of mouse KOR gene activities. Collectively, enhanced binding of Sp1 to promoter I of the KOR gene as a result of inhibiting the ERK pathway contributes to RA induction of this gene in P19.

SUMO-1 modification represses Sp3 transcriptional activation and modulates its subnuclear localization

The GC box binding transcription factor Sp3 both activates and represses transcription. We have found that Sp3 activity is regulated by SUMO-1 modification. Endogenous Sp3 is sumoylated and localized to the nuclear periphery and in nuclear dots. Removal of SUMO-1 from Sp3 by mutation of the SUMO acceptor lysines or expression of the SUMO-1 protease SuPr-1 converted Sp3 to a strong activator with a diffuse nuclear localization. Covalent attachment of SUMO-1 to Sp3 by gene fusion was sufficient to repress Sp3-dependent transcription and relocalize Sp3 to the nuclear periphery and nuclear dots. These studies reveal a direct effect of SUMO-1 modification on activity of a dual function transcription factor and provide a mechanism for functional specificity within the Sp transcription factor family.

Transcription factor Sp3 is silenced through SUMO modification by PIAS1

Sp3 is a ubiquitous transcription factor closely related to Sp1. Here we show that Sp3 is a target for SUMO modification in vivo and in vitro. SUMO modification of Sp3 occurs at a single lysine located between the second glutamine-rich activation domain and the DNA-binding domain. Mutational analyses identified the sequence IKXE as essential for SUMO conjugation to Sp3. We identified the protein inhibitor of activated STAT1 (PIAS1) as an interaction partner of Sp3 and Ubc9. Moreover, PIAS1 strongly stimulated SUMO conjugation to Sp3, thus acting as an E3 ligase for SUMO conjugation to Sp3. All mutations that prevented SUMO modification in vitro strongly enhanced the transcriptional activity of Sp3, showing that SUMO modification silences Sp3 activity. SUMO-modified Sp3 bound to DNA with similar specificity and affinity as unmodified Sp3. However, DNA-bound Sp3 did not act as a substrate for SUMO modification.

Regulation of the activity of Sp1-related transcription factors

The initiation of transcription is accomplished via interactions of many different proteins with common and gene-specific regulatory motifs. Clearly, sequence-specific transcription factors play a crucial role in the specificity of transcription initiation. A group of sequence-specific DNA-binding proteins, related to the transcription factor Sp1, has been implicated in the regulation of many different genes, since binding sites for these transcription factors (GC/GT boxes) are a recurrent motif in regulatory sequences such as promoters, enhancers and CpG islands of these genes. The simultaneous occurrence of several homologous GC/GT box-binding factors precludes a straightforward deduction of their role in transcriptional regulation. In this review, we focus on the connection between functional specificity and biochemical properties including glycosylation, phosphorylation and acetylation of Sp1-related factors.

Sp3 expression in immune cells: a quantitative study

We previously reported the lack of expression of the bifunctional transcription factor Sp3 in peripheral blood mononuclear cells from most patients with multiple sclerosis (MS) (Grekova et al, 1996). An RT-PCR technique was developed to evaluate Sp3 mRNA levels in peripheral blood mononuclear cell subsets. Semi-quantitative and quantitative competitive RT-PCR assays were used to compare the level of Sp3 expression among subjects and among immune cell subsets. The competitor DNA fragment contained a deletion from the normal Sp3 cDNA sequence. The wild-type Sp3 cDNA and the competitor DNA fragment amplified with equal efficiency, and the two PCR products were distinguished by size. These studies demonstrated that normal CD4(+) and CD8(+) T cells, B cells, and macrophages expressed comparable amounts of Sp3 mRNA. No Sp3 expression could be detected in normal natural killer cells nor in any of these cell types from Sp3-negative MS patients. We propose that transcription of the Sp3 gene is blocked in immune cells from most patients with MS and that this contributes to the development of central nervous system inflammation in the disease.

Complex regulation of human neuronal nitric-oxide synthase exon 1c gene transcription. Essential role of Sp and ZNF family members of transcription factors

Neuronal nitric-oxide synthase (nNOS) is expressed in a variety of human tissues and shows a complex transcriptional regulation with the presence of nine alternative first exons (1a-1i) resulting in nNOS transcripts with differing 5'-untranslated regions. We previously demonstrated that nNOS exon 1c, one of the predominant transcripts in the human gastrointestinal tract, is driven by a separate promoter (Saur, D., Paehge, H., Schusdziarra, V., and Allescher, H. D. (2000) Gastroenterology 118, 849-858). The present study focused on the quantitative expression of nNOS first exon variants in different human tissues and the characterization of the basal nNOS exon 1c promoter. In human brain, skeletal muscle, colon, and TGW-nu-I neuroblastoma cells, first exon expression patterns were analyzed by quantitative real-time reverse transcription-PCR. In these tissues/cells exon 1c was one of the most abundant first exons of nNOS. By transient transfections of TGW-nu-I and HeLa cells with reporter plasmids containing a series of 5' and 3' deletions in the exon 1c regulatory region, the minimal TATA-less promoter was localized within 44 base pairs. Gel mobility shift assays of this cis-regulatory region revealed a high complexity of the basal promoter with a cooperative binding of several transcription factors, like Sp and ZNF family members. When the Sp binding site of the minimal promoter construct was mutated, promoter activity was completely abolished in both cell lines, whereas mutation of the common binding site of ZNF76 and ZNF143 resulted in a decrease of 53% in TGW-nu-I and 37% in HeLa cells. In Drosophila Schneider cells expression of Sp1, the long Sp3 isoform, ZNF76 and ZNF143 potently transactivated the nNOS exon 1c promoter. These results identify the critical regulatory region for the nNOS exon 1c basal promoter and stress the functional importance of multiple protein complexes involving Sp and ZNF families of transcription factors in regulating nNOS exon 1c transcription.

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

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

Formation of the head-trunk boundary in the animal body plan: an evolutionary perspective

Gene expression analyses and anatomical studies suggest that the body plans of protostomes and deuterostomes are phylogenetically related. In the central nervous system (CNS), arthropods and vertebrates (as well as their closest related phyla the urochordates and cephalochordates) share a nerve cord with rostral specification: the cerebral neuromeres in Drosophila, cerebral sensory vesicle of ascidians and lancelets and the large brain of craniates. Homologous genes, in particular of the otd/Otx and Hox families, are at play in these species to specify the anterior and posterior CNS territories, respectively. In contrast, homologies in the establishment of boundary regions like those separating head and trunk structures in arthropods or mid- and hindbrain domains in chordates are still unclear. We compare in these species the formation, properties and molecular characteristics of these boundaries during embryonic development. We also discuss recent findings suggesting that insects and vertebrates might have co-opted factors of related families to control the formation of these boundary regions, the evolution of which would then appear dramatically different from that of the anterior and posterior CNS domains.

Sp3 represses gene expression via the titration of promoter-specific transcription factors

We have determined previously that Sp3 encodes three distinct gene products as follows: a full-length protein (Sp3) that is an activator of transcription and two isoforms (M1 and M2) derived via internal translational initiation that function as transcriptional repressors. To identify amino acids and functions required for transcriptional repression, we employed PCR-directed mutagenesis to create a panel of mutated M2 proteins. Biochemical and functional analyses of these mutated proteins indicate that functions encoded by the M2 carboxyl terminus, such as DNA binding activity and the capacity to form multimeric complexes, are not required or sufficient for transcriptional repression. Instead, a 93-amino acid portion of the trans-activation domain was shown to be the minimal portion of M2 required to block Sp-dependent gene expression. Transcriptional analysis of three Sp-dependent promoters showed that mutations sustained by many M2 proteins result in promoter-specific effects. Regions of M2 required for physical interactions with five TATA box-associated factors (TAF(II)s) were mapped, and mutations that disrupt the interaction of M2 with two of these proteins, TAF(II)70 and TAF(II)40, were identified. We conclude that Sp3- mediated transcriptional repression is due, at least in part, to competition for promoter-specific transcription factors.

GC-rich elements flanking the transcription start site govern strong activation on the SNURF gene

To study the regulation of the murine small nuclear RING finger protein SNURF (RNF4) gene, approximately 0.7 kb of its TATA-less promoter was isolated. This fragment conferred strong activation in reporter gene assays, yielding > or = 30% of the activity of the SV40 virus promoter/enhancer construct. Interestingly, the short region from -38 to +36 flanking the transcription start site was sufficient for potent basal promoter activity in various mammalian cell lines. Mutation of the conserved GC box at +9 abolished nuclear protein binding to the proximal promoter and severely compromised promoter activity, suggesting that this element is critical for the assembly of the transcription apparatus to regulate SNURF gene expression. Furthermore, our results show that the Wilms' tumor 1 gene product is one of the potential activators of the SNURF gene.