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

The promoter activity of long terminal repeats of the HERV-H family of human retrovirus-like elements is critically dependent on Sp1 family proteins interacting with a GC/GT box located immediately 3' to the TATA box

The HERV-H family of endogenous retrovirus-like elements is widely distributed in the human genome, with about 1,000 full-length elements and a similar number of solitary long terminal repeats (LTRs). HERV-H LTRs have been shown to direct the transcription of both HERV-H-encoded and adjacent cellular genes. Transcripts of HERV-H elements are especially abundant in placenta, teratocarcinoma cell lines, and cell lines derived from testicular and lung tumors. Here we report that only a subset of HERV-H LTRs display promoter activity in human cell lines and that these LTRs are characterized by the presence of a GC/GT box immediately downstream of the TATA box. This GC/GT box is required for promoter activity, while, surprisingly, the TATA box is dispensable. The ubiquitously expressed transcription factors Sp1 and Sp3 bound to this GC/GT box and stimulated transcription from the promoter-active LTRs in the teratocarcinoma cell line NTera2-D1. However, in HeLa and Drosophila SL-2 cells, Sp1 acted as a transcriptional activator of the LTRs, while Sp3 acted as a repressor of Sp1-mediated transcriptional activation. Cotransfection studies also revealed that the tissue-specific Sp1-related protein BTEB bound to this GC/GT box and stimulated transcription from the LTR promoters in NTera2-D1 cells. These results show that members of the Sp1 protein family are crucial determinants for transcriptional activation of HERV-H LTR promoters and suggest that these proteins may also be involved in determining the tissue-specific expression pattern of HERV-H elements.

Characterization of the human alpha 1(VI) collagen promoter and its comparison with human alpha 2(VI) promoters

From a human cosmid library, we isolated a clone (5B) with an insert of 32 kb, encoding the amino-terminal and the 5'-end flanking region of the alpha 1(VI) collagen gene. Exon 1 was found to be 194 bp and contain the 5' untranslated region plus 97 bp coding sequence. Exon 2 consists of 130 bp, a size that is conserved across the chicken and mouse species. S1-nuclease-protection assays and primer-extension analysis, using mRNA from human dermal fibroblasts, show the presence of multiple transcription start sites located in a region of approximately 20 nucleotides. Canonical TATA and CAAT boxes, as found in the chicken and mouse alpha 1 promoters, were absent in the human alpha 1(VI) promoter. The promoter region from positions -1 to -190, is a polypyrimidine/polypurine-rich region containing 12 CCCTCCCC (CT element consensus) sequences and has multiple potential binding sites for the Sp1, and AP2 transcription factors. These regulatory proteins bind to the alpha 2(VI) promoters [Saitta, B. & Chu, M.-L. (1994) Eur. J. Biochem. 223, 675-682]. To test the transcriptional activity of the alpha 1 promoter, transient transfection experiments of the DNA constructs were performed in human dermal fibroblasts and in human fibrosarcoma (HT1080) cell lines. The DNA constructs drive the expression of the chloramphenicol acetyl transferase (CAT) gene. The results show strong CAT activity for the constructs at positions -1700, -298 and -257, while low activity was found for the constructs at positions -4400, -142 and -5 when transfected in fibroblasts. The experiments also identified positive and negative regulatory regions in the alpha 1(VI) promoter CAT constructs when transfected in fibroblasts, but did not identify them in the fibrosarcoma cells.

Identification of Sp1-binding sites in the CD11c (p150,95 alpha) and CD11a (LFA-1 alpha) integrin subunit promoters and their involvement in the tissue-specific expression of CD11c

The leukocyte integrins LFA-1 (CD11a/CD18) and p150,95 (CD11c/CD18) mediate cell-cell and cell-extracellular matrix interactions during inflammatory responses and signal transduction into the cytoplasm. While the CD11a integrin subunit is expressed on all leukocytes, CD11c is almost exclusively expressed on cells of the myeloid lineage and on activated B lymphocytes. Its expression is regulated during cell activation and differentiation by transcriptional mechanisms. We have previously demonstrated that the proximal region of the CD11c promoter directs tissue-restricted and developmentally-regulated expression of reporter genes. Structural studies by electrophoretic mobility shift assays have demonstrated the presence of two Sp1-binding sites at -70 (Sp1-70) and -120 (Sp1-120) which mediate the Sp1 transactivation of the CD11c promoter in Sp1-defective SL2 cells, and which are involved in cell lineage-specific DNA-protein interactions, as demonstrated by footprinting in vivo. More importantly, mutation of either Sp1 site inhibited the activity of the CD11c promoter both in myeloid U937 cells and the CD11c-expressing B lymphoblastoid JY cell line, while the opposite effect was observed in the CD11c-negative epithelial HeLa cell line, demonstrating the involvement of both Sp1-binding sites in the basal and the tissue-restricted expression of the CD11c integrin subunit gene. Interestingly, the analysis of the CD11a proximal promoter also revealed the existence of an Sp1-binding site at -70, indicating a common role for these cis-acting elements in the transcription of the leukocyte integrin alpha subunit genes. The binding of Sp1 to the regulatory regions of the leukocyte integrin genes raises the possibility that the retinoblastoma susceptibility gene product is implicated in integrin expression through its functional interaction with Sp1, thus establishing a link between integrin-dependent leukocyte adhesiveness and the state of cellular differentiation/proliferation.

Functional analysis of the transforming growth factor beta responsive elements in the WAF1/Cip1/p21 promoter

The transforming growth factor beta s (TGF-beta s) are a group of multifunctional growth factors that inhibit cell cycle progression in many cell types. The TGF-beta-induced cell cycle arrest has been partially attributed to the regulatory effects of TGF-beta on both the levels and activities of the G1 cyclins and their cyclin-dependent kinase partners. The ability of TGF-beta to inhibit the activity of these kinase complexes derives in part from its regulatory effects on the cyclin-dependent kinase inhibitors, p21/WAF1/Cip1, p27Kip1, and p15. Upon treatment of cells with TGF-beta, these three inhibitors bind to and block the activities of specific cyclin-cyclin-dependent kinase complexes to cause cell cycle arrest. Little is known, however, on the mechanism through which TGF-beta activates these cyclin-dependent kinase inhibitors. In the case of p21, TGF-beta treatment leads to an increase in p21 mRNA. This increase in p21 mRNA is partly due to transcriptional activation of the p21 promoter by TGF-beta. To further define the signaling pathways through which TGF-beta induces p21, we have performed a detailed functional analysis on the p21 promoter. Through both deletion and mutation analysis of the p21 promoter, we have defined a 10-base pair sequence that is required for the activation of the p21 promoter by TGF-beta. In addition, this sequence is sufficient to drive TGF-beta-mediated transcription from a previously nonresponsive promoter. Preliminary gel shift assays demonstrate that this TGF-beta responsive element binds specifically to several proteins in vitro. Two of these proteins are the transcription factors Sp-1 and Sp-3. These studies represent the initial steps toward defining the signaling pathways involved in TGF-beta-mediated transcriptional activation of p21.

A CCACC motif mediates negative transcriptional regulation of the human erythropoietin receptor

We have previously shown that the +79 to +135 fragment of the human erythropoietin receptor (Epo-R) acts negatively on the transcriptional activity and confers erythroid specificity to the gene [Maouche, L., Cartron, J.-P. & Chrétien, S. (1994) Nucleic Acids Res. 22, 338-346]. In this work, we demonstrate that this effect is mediated by a CCACC motif that binds weakly to the simian virus 40 protein 1 (Sp1) factor and that the increase of the affinity for Sp1 augments transcription inhibition. The repression is not restricted to the human Epo-R promoter, although it seems more efficient on heterologous promoters of erythroid genes. In chloramphenicol acetyl transferase constructs containing the mouse Epo-R promoter, rearranged by retroviral long terminal repeat (LTR) insertion of murine erythroleukemia cell lines, we found that positioning the CCACC motif 3' to the LTR represses the transcriptional activity mediated by the LTR in non-erythroid cells. These results demonstrate that Epo-R gene expression is negatively regulated by a CCACC or a GC box-binding factor, which is most likely identical to the Sp1 transcription protein. Further data suggest that Sp1-mediated negative regulation is not the result of a direct competition between Sp1 and another DNA-binding protein.

Structural and functional analysis of the promoter of the murine V gamma 1.1 T cell receptor gene

The expression of the germ-line gene V gamma 1.1-C gamma 4 of the T cell receptor (TcR) gamma chain depends on interleukin (IL)-3 induction in hematopoietic cells, while in T cells, the rearranged gene is expressed constitutively. To understand the mechanism that controls TcR gamma gene expression, we cloned and characterized the structure and function of the V gamma 1.1-C gamma 4 TcR promoter. IL-3-dependent cell lines and T cell lines utilized the same transcriptional start sites. In chloramphenicol acetyltransferase (CAT) assays, the minimal 70-bp promoter confers strong transcriptional activity which is 50-60% of the Moloney long terminal repeat promoter activity. The 500-bp promoter region linked to the CAT gene exhibits IL-3 dependency similar to the endogenous TcR gamma gene. The immediate 3' and 5' flanking sequences inhibit the promoter activity two- to fourfold. The promoter lacks an obvious TATA box or CAAT box sequences, but contains a GC box in the untranslated region 3' to the promoter. The GC box is the core sequence of the element which binds Sp1-like proteins. Cloning of this Sp1 binding element in front of the thymidine kinase (TK) promoter and mutations generated in this site demonstrate its function as a silencer. Ultraviolet cross-linking analysis with the Sp1 binding site from the TcR gamma promoter revealed binding of a 90-100-kDa protein in a T cell line (EL-4) and 40-50 and 90-100-kDa proteins in FDC-P1 cells. The possible function of the Sp1-like protein in silencing the minimal promoter activity is discussed.

Transcriptional modulators affect in vivo protein binding to the low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl coenzyme A reductase promoters

Treatment of HepG2 cells with known effectors of low density lipoprotein receptor (LDLR) gene expression altered the in vivo pattern of protein-DNA interactions in the promoter. The observed changes are consistent with proteins binding in vivo to the sterol regulatory element (SRE), to Sp1-like sites, as well as to other regions. Protein bound to the SRE in all conditions, but the nature of the dimethyl sulfate reactivity changed depending on the physiological state of the cell. Hypermethylation within the SRE of the low density lipoprotein receptor promoter was observed when cells were treated with cholesterol synthesis inhibitors, insulin, or phorbol 12-myristate 13-acetate, suggesting that the SRE regulates this promoter through sterol-independent as well as sterol-dependent mechanisms. No significant changes were observed in binding to the Sp1-like sites, suggesting that differential binding to these sites does not play a role in altered transcription levels. Analysis of the 3-hydroxy-3-methylglutaryl coenzyme A reductase promoter also revealed protections that varied in a cell type-specific manner. Binding to the 3-hydroxy-3-methylglutaryl coenzyme A reductase SRE and putative nuclear factor 1 sites could be observed but varied little in different physiological conditions.

Sp1 is phosphorylated and its DNA binding activity down-regulated upon terminal differentiation of the liver

Using nuclear extracts prepared from rat liver it was demonstrated that binding of a transcription factor to site II of the D-site binding protein promoter could be induced by dephosphorylation of these extracts. Competition band shifts and supershift assays reveal this protein to be the general transcription factor Sp1. Phosphorylation of Sp1 appears to occur as a result of terminal differentiation of the liver. Proteins from both 1-day-old rat liver and adult liver undergoing regeneration have less of the phosphorylated form of Sp1 present with consequent increased DNA binding activity. Sp1 is similarly phosphorylated in brain, kidney, and spleen with phosphatase treatment of the extracts significantly increasing the level of DNA binding activity. Dephosphorylation of Sp1 results in a 10-fold increase in the affinity of Sp1 for its cognate site. Two-dimensional gel electrophoresis reveals that approximately 20% of the detectable protein appears to be in the phosphorylated form in adult liver extracts. Another protein with similar characteristics also appears to be present in the liver. Decreasing Sp1 DNA binding activity by phosphorylation may be an important mechanism for regulating gene expression, and possibly bringing about growth arrest during terminal differentiation.

Functional analyses of the transcription factor Sp4 reveal properties distinct from Sp1 and Sp3

Sp4 is a human sequence-specific DNA binding protein with structural features similar to those described for the transcription factors Sp1 and Sp3. These three proteins contain two glutamine-rich regions and a highly conserved DNA binding domain composed of three zinc fingers. Consistently, Sp1, Sp3, and Sp4 do have the same DNA binding specificities. In this report, we have embarked on a detailed analysis of the transcriptional properties of Sp4 in direct comparison to Sp1 and Sp3. Cotransfection experiments into Drosophila SL2 cells lacking endogenous Sp factors demonstrate that Sp4 is an activator protein like Sp1. However, in contrast to Sp1, Sp4 is not able to act synergistically through adjacent binding sites. The transactivation function of Sp4 resides, like that of Sp1, in the N-terminal glutamine-rich region. Sp4 can function as a target for the Sp1 activation domains in a superactivation assay, suggesting that the activation domains of Sp1 and Sp4 are functionally related. Furthermore, we show that Sp4-mediated transcriptional activation can be repressed by Sp3. Taken together, our results demonstrate that the transcription factor Sp4 exhibits specific functional properties distinct from Sp1 and Sp3.

A novel transcription factor and two members of the Sp 1 multigene family regulate the activity of the alpha 2 (VI) collagen promoter

For a better understanding of the processes that lead to the activation or inhibition of type VI collagen synthesis, we have identified and characterized the cis-acting elements of the chicken alpha 2 (VI) collagen promoter. This promoter encompasses four sites, termed S1, S2, X and S3, which interact with nuclear factors. Sites S1, S2 and S3 are each recognized by the same two proteins that belong to the Sp 1 multigene family. Site X appears to interact with a novel transcription factor of 43 kDa. When a fragment containing all four of the elements is placed in front of a reporter gene, the resulting construct is able to induce a high level of transcription in chicken fibroblasts. As soon as a single element is deleted from this construct, the activity decreases drastically. Thus, the four elements are essential for the transcriptional activation of the alpha 2 (VI) collagen gene.

A TATA-less promoter containing binding sites for ubiquitous transcription factors mediates cell type-specific regulation of the gene for transcription enhancer factor-1 (TEF-1)

TEF-1 is a tissue-specific human transcription factor which binds to and activates transcription from the SV40 early promoter and the HPV-16 E6/E7 promoter and may be involved in regulation of muscle-specific and placenta-specific gene expression. To investigate the mechanism of its tissue-specific expression, we have isolated up to 3 kilobase pairs of 5'-flanking DNA and characterized the promoter of the gene for TEF-1. Multiple transcription start sites centering on a motif similar to the initiator element (Inr) were identified. A minimal promoter, which contains no recognizable TATA element but contains an Inr, delimited at -137 base pairs had full transcriptional activity both in vivo in HeLa cells and in vitro in HeLa cell extracts. This promoter is also highly active in vitro in lymphoid cell extracts, but not in vivo in lymphoid cell lines, which do not express the endogenous TEF-1 gene. The minimal promoter, which is sufficient to direct tissue-specific expression of the TEF-1 gene in vivo, contains multiple sites which bind the ubiquitous transcription factors Sp1 and ATF-1. Mutation of the Inr completely abolished transcription from the major start site while transcription from the minor sites was slightly augmented. Inactivation of the proximal Sp1 site abolished transcription from the principle start site and increased transcription from a 5' minor start site. Insertion of a TATA box element did not qualitatively alter the pattern of start site usage which seemed to be dependent upon integrity of the upstream Sp1 site. These observations suggest a "cross-talk" between the Inr and a proximal element to fix transcription start sites, which is independent of spacing and the presence of a TATA element.

Structural and functional analysis of the human KB cell folate receptor gene P4 promoter: cooperation of three clustered Sp1-binding sites with initiator region for basal promoter activity

The human folate receptors (hFRs) are important in the cellular accumulation of folates and antifolates. We described the structure of the human KB cell FR (hFR-KB) gene and identified two discrete promoter regions (P1 and P4) upstream from exons 1 and 4, respectively (Elwood et al., 1993). To further understand the molecular basis of hFR expression, we have now analyzed the basal transcription of the P4 promoter localized upstream of a major transcription start site. The sequence upstream from exon 4 contains several potential transcriptional factor-binding sites and a consensus initiator region sequence at the transcription start site but does not contain canonical TATA or CAAT boxes. While deletion of a 5' flanking sequence from nt -1023 to nt -605 of P4 promoter region decreases the luciferase reporter gene expression in KB cells to 54-70% of control construct, the removal of the sequence between nt -292 and nt -46 markedly decreases the activity to 3%. DNase I footprints and competitive mobility shift and supershift mobility assays indicate that Sp1 or Sp1-related nuclear protein(s) bind to three clustered GC-rich regions within the sequence between nt -292 and nt -46 of the hFR-KB P4 promoter. Both in vitro and in vivo analyses of the expression of promoter constructs containing site-specific mutation(s) of these three Sp1-binding sites and initiator sequence demonstrate that each of three Sp1 sites and the initiator sequence are required for optimum promoter activity and that they interact cooperatively in this P4 promoter of the hFR-KB gene.

The human medium chain Acyl-CoA dehydrogenase gene promoter consists of a complex arrangement of nuclear receptor response elements and Sp1 binding sites

Expression of the gene encoding the mitochondrial fatty acid. beta-oxidation enzyme, medium-chain acyl-CoA dehydrogenase (MCAD), is regulated among tissues during development and in response to alterations in substrate availability. To identify and characterize cis-acting MCAD gene promoter regulatory elements and corresponding transcription factors, DNA-protein binding studies and mammalian cell transfection analyses were performed with hjman MCAD gene promoter fragments. DNA:protein binding studies with nuclear protein extracts prepared from hepatoma G2 cells, 3T3 fibroblasts, or Y-1 adrenal tumor cells identified three sequences (nuclear receptor response element 1 or NRRE-1, NRRE-2, and NRRE-3) that bind orphan members of the steroid/thyroid nuclear receptor superfamily including chicken ovalbumin upstream promoter transcription factor and steroidogenic factor 1. Sp1 binding sites (A-C) were identified in close proximity to each of the NRREs. NRRE-3 conferred cell line-specific transcriptional repression by interacting with chicken ovalbumin upstream promoter transcription factor or activation via steroidogenic factor 1. In contrast, the Sp1 binding site A behaved as a transcriptional activator in all cell lines examined. We propose that multiple nuclear receptor transcription factors interact with MCAD gene promoter elements to differentially regulate transcription among a variety of cell types.

Overexpression of phospholipase C-gamma 1 in colorectal carcinomas is associated with overexpression of factors that bind its promoter

The 5'-upstream sequence of the phospholipase C-gamma 1 (PLC-gamma 1) gene contains several transcriptional regulatory regions. We have studied one of the regions (-551 to -480, named GPE1) which exhibits a strong positive regulatory activity. GPE1 stimulated the transcription when fused to heterologous TATA element in an orientation-dependent manner. The region between -536 and -470 was identified as the protein binding site in GPE1 by the DNase I footprinting method. Electrophoretic mobility shift assays with several competitors revealed three protein binding sites in this region, designated as GES1, GES2, and GES3. The binding sites were -535 GGAGGGGGCG -524, -512 TGTCACTCA -504, and -491 CAATCCA -485, respectively. Mutational analyses suggested that GPE1 binding proteins cooperate with each other to activate the transcription of the PLC-gamma 1 gene. Additionally, immunoblot analyses revealed that the level of PLC-gamma 1 expression was considerably higher in 9 of 11 colorectal carcinomas than in adjacent normal colorectal tissues. In 7 of 9 cases of colorectal carcinomas which express higher level of PLC-gamma 1, the DNA binding activities to GES1, GES2, and GES3 sites also increased when compared with normal tissues. These results suggest that the GPE1 binding proteins might be attributed to the elevated expression of PLC-gamma 1 in colorectal carcinomas and may play important roles in proliferation of colorectal carcinoma cells.

Transcriptional activation of the neuronal peripherin-encoding gene depends on a G + C-rich element that binds Sp1 in vitro and in vivo

Peripherin (Prph) is a type-III intermediate filament (IF) protein principally synthesized in peripheral nervous system neurons. We have previously shown that three regulatory elements, PER1, PER2 and PER3, in the first 98 bp of the Prph gene promoter, were sufficient to direct cell-type specific expression of a reporter gene [Desmarais et al., EMBO J. 11 (1992) 2971-2980]. Of these elements, PER1 was found to be important for cell-type specificity, but required the presence of other elements for transcriptional activity. Here, we show that PER3 is a stronger activator than PER2 and that it can stimulate cell-type-specific transcription when combined with PER1. We have characterized the G + C-rich PER3 element for its ability to bind trans-acting factors. Gel retardation and methylation interference (MI) assays show that PER3 binds transcription factor Sp1. In addition, an anti-Sp1 antibody recognizes the PER3 DNA-binding protein. A 3-bp mutation abrogating the capacity of PER3 to bind Sp1 in vitro completely abolished expression of the reporter gene construct containing only PER3 and PER1, while in a construct containing the first 256 bp of the Prph promoter, it led to an 80% decrease with respect to the control wild-type construct. Finally, by co-transfection of a Sp1-expressing plasmid, we show that Sp1 can stimulate transcription from a reporter gene containing the PER3 sequence. Together, these results indicate that interactions between Sp1 and the proteins binding PER1 are involved in the control of the Prph gene.

HIV Tat represses transcription through Sp1-like elements in the basal promoter

MHC class I genes are potently repressed by HIV Tat, which transactivates the HIV LTR. Tat represses class I transcription by binding to complexes associated with a novel promoter element, consisting of Sp1-like DNA binding sites. Transcription by other Sp1-dependent promoters, such as MDR1 and the minimal SV40 promoters, is also repressed by Tat, whereas the human beta-actin promoter is neither activated by Sp1 nor repressed by Tat. Tat repression can be overcome by a strong enhancer element. Thus, the SV40 72 bp enhancer element confers protection from Tat-mediated repression on both the minimal SV40 promoter and the class I promoter. Surprisingly, Tat can activate the class I promoter in the presence of both the HIV TAR element and a strong upstream enhancer. These data demonstrate that Tat differentially affects Sp1-responsive promoters, depending on promoter architecture.

Identification of two distinct Sp1- and RBF-1-like nuclear factors that bind to the upstream region of the human angiotensinogen promoter

Angiotensionogen, the protein precursor of angiotensin II that is a crucial regulator of blood pressure and electrolyte balance, is constitutively produced by the liver. In the present study, we identified two nuclear factors that are possibly involved in maintaining the constitutive promoter activity of the human angiotensinogen gene. The 32 bp DNA region between -344 and -313 located in the 1.3 kb angiotensinogen upstream region (-1222 to +44) partially contributed to the maintenance of the efficient promoter activity in HepG2 cells. This segment was able to form the complexes with HepG2 nuclear extracts, which could be dissociated by competing recognition sequences that contain those of either Sp1 or RBF-1. Anin vivo competition experiment demonstrated that the parental promoter activity is reduced about 65% by an RBF-1 competitor more effectively than by an Sp1 competitor. These results suggested that Sp1- and RBF-1-like factors play roles in maintaining the constitutively active angiotensinogen promoter.

Functional analysis of the human endothelial nitric oxide synthase promoter. Sp1 and GATA factors are necessary for basal transcription in endothelial cells

To gain insights into the mechanisms of endothelial nitric oxide synthase (eNOS) gene expression, we have cloned the eNOS promoter and fused it to a luciferase reporter gene to map regions of the promoter important for basal transcription in bovine aortic endothelial cells (BAEC). Transfection of BAEC with F1 luciferase (LUC) (-1600 to +22 nucleotides) yielded a 35-fold increase in promoter. Progressive deletion from -1600 to -1033 (F2 and F3 LUC) did not significantly influence eNOS promoter activity. Further deletion from -1033 to -779 (F4 LUC) resulted in an approximate 40% reduction in basal promoter activity, and still further deletion from -779 to -494 (F5 LUC) did not markedly influence activity. Deletion from -494 to -166 (F6 LUC) reduced eNOS promoter activity by 40-50%. Specific mutation of the consensus GATA site (-230) in the F3 LUC construct reduced luciferase activity (by 25-30%). Gel shift analysis and antibody depletion using BAEC nuclear extracts demonstrated in vitro binding of GATA-2 to the oligonucleotide sequence containing the -230 GATA site. Next, we mutated the Sp1 site (-103) in the F3 and F6 LUC constructs and in the F3 GATA mutant construct. Expression of these Sp1 mutants in BAEC resulted in a 85-90% reduction in normalized luciferase activity. Gel shift and antibody supershift analysis using a BAEC nuclear extracts demonstrated four specific, DNA-protein complexes binding to the eNOS Sp-1 site, with the slowest migrating form composed of Sp1 and another nuclear protein. These data demonstrate that the Sp1 site is an important cis-element in the core eNOS promoter.

Members of the Sp transcription factor family control transcription from the uteroglobin promoter

Previous analyses of the uteroglobin promoter revealed seven distinct regions, which contribute to its overall activity in epithelial cells from endometrium and lung. Most significantly, a mutation of the promoter sequence around 65 base pairs upstream of the transcriptional start site severely impairs promoter activity. The transcription factor acting through this sequence has not been identified yet. Here, we report that members of the Sp transcription factor family specifically recognize this non-classical GC box, in addition to another functional motif located 230 base pairs upstream of the transcriptional start site. We have characterized in detail the interaction of recombinant Sp3 with both motifs by DNase I footprinting and methylation protection using the wild-type uteroglobin promoter and various linker scanning mutants as templates. Electrophoretic mobility shift analyses show that Sp1 and Sp3 both bind with similar affinity to these elements. We demonstrate that the DNA-binding proteins in the endometrial cell line Ishikawa which recognize these motifs are also Sp1 and Sp3. Gene transfer experiments into Drosophila Schneider cells that do not contain endogenous Sp factors revealed that both DNA motifs respond to transiently expressed Sp1 and Sp3. Our results show thus that the level of transcription from the uteroglobin promoter is controlled by members of the Sp transcription factor family through unusual Sp binding sites.

Cell-specific function of cis-acting elements in the regulation of human alcohol dehydrogenase 5 gene expression and effect of the 5'-nontranslated region

The human alcohol dehydrogenase 5 gene (ADH5) differs from all other human alcohol dehydrogenase genes in its ubiquitous expression, although there are tissue-specific differences in the level of expression. To understand the expression of ADH5, we characterized the structure and function of its 5' region by DNase I foot-printing and transient transfection assays. The region from base pair (bp) -34 to +61, flanking the major transcription start site, had strong promoter activity in three different cell lines: HeLa, H4IIE-C3, and CV-1, and could explain the ubiquitous expression. Two Sp1 sites within that region are footprinted by nuclear extracts from all tissues and cells tested. There are sites further upstream that show cell- and tissue-specific differences in both their patterns of occupancy and their effects on promoter activity. The region between bp -34 and -64 strongly increases promoter activity in H4IIE-C3 cells, weakly activates in CV-1 cells, but has no effect in HeLa cells. The region between bp -127 and -163 is a positive element in both HeLa cells and CV-1 cells, but is a negative regulatory element in H4IIE-C3 cells. These differences in part explain the levels of expression of ADH5 in various tissues. Two regions (bp -64 to -127 and bp -163 to -365) contain negative regulatory elements that reduce promoter activity in all three cells. The 5'-nontranslated region of ADH5 contains two upstream ATGs. Insertion of 12 bp within the putative coding region of these upstream ATGs led to a 1.6-2.3-fold increase in activity. This suggests that the 5'-nontranslated region has regulatory significance.

Transcriptional analysis of acetylcholine receptor alpha 3 gene promoter motifs that bind Sp1 and AP2

In this study, we performed an analysis of the neuronal nicotinic acetylcholine receptor alpha 3 subunit gene promoter region, -238/+47, to identify cis and trans elements that are important for basal activity in PC12 cells. Sequence analyses of the alpha 3 promoter and footprint assays revealed an Sp1 binding site between -79 and -57 (termed the alpha 3 GA motif) and an AP2 binding site between -30 and -7. Using mobility shift analysis, we found that PC12 cell extracts contain proteins that specifically bind to the alpha 3 GA motif and are immunologically related to Sp1. Mutation of the alpha 3 GA motif, which prevented binding of Sp1, resulted in a 75% decrease in promoter activity. Mutation of the AP2 site resulted in only a minor loss of promoter activity, which is consistent with the lack of AP2 binding activity in PC12 extracts. In Drosophila Schneider line 2 (S2) cell cotransfection assays, Sp1 activated the alpha 3 promoter in a GA motif-dependent manner. Furthermore, multimerization of the GA motif upstream of the beta-globin TATA box conferred Sp1 responsiveness. Our results indicate that Sp1 can activate transcription through direct interaction with the alpha 3 GA motif and that this motif plays a major role in alpha 3 promoter basal activity in PC12 cells.

Mutually exclusive interactions between factors binding to adjacent Sp1 and AT-rich elements regulate gastrin gene transcription in insulinoma cells

The gastrin gene is transiently expressed in fetal pancreatic islets during islet neogenesis but then switched off after birth when islet cells become fully differentiated. Previous studies identified a cis-regulatory sequence between -109 and -75 in the human gastrin promoter which binds islet cell-specific activators and a nonspecific repressor and thus may act as a molecular switch. The present study identified another cis-regulatory sequence (-163ACACTAAATGAAAGGGCGGGGCAG-140) which bound two islet nuclear proteins in a mutually exclusive manner, as defined by gel shift competition, methylation interference, and DNase I foot-printing assays. The general transactivator Sp1 recognized the downstream GGGCGGGG sequence, but Sp1 binding was prevented when another islet factor bound to the adjacent AT-rich sequence (CTAAATGA). This gastrin AT-rich element is nearly identical to the binding site (ATAAATGA) for the islet-specific transcription factor beta TF-1. However, the gastrin AT-binding factor appeared to differ from beta TF-1 in its gel mobility shift pattern. Transfections of rat insulinoma cells revealed that mutations which blocked binding to the AT-rich element but allowed Sp1 binding up-regulated transcriptional activity. These results suggest that the gastrin AT-binding factor blocks transactivation by Sp1 and may have a role in the repression of gastrin transcription seen at the end of islet differentiation.

Functional analysis of the human interleukin 2 receptor gamma chain gene promoter

The third component of the interleukin (IL) 2 receptor, gamma chain, is essential not only for IL-2- but also for IL-4-, IL-7-, IL-9-, and IL-15-induced proliferation of lymphocytes. To elucidate the mechanisms by which the gamma chain is expressed, we have analyzed the promoter region of the gamma chain gene. The 633-base pair fragment upstream of the initiation codon showed the promoter activity in human hematopoietic cell lines, Jurkat and THP-1, when linked to the luciferase gene. With a series of 5'-deletion mutants, the basal promoter activity was found in a fragment from nucleotide 80 to 58 upstream from the RNA start site, including an Ets binding sequence. Treatment of cells with either 12-O-tetradecanoylphorbol-13-acetate or phytohemagglutinin but not forskolin induced transcription from the gamma chain gene promoter. A viral trans-acting transcriptional activator, Tax, of human T-cell leukemia virus type I elevated expression of the gamma chain gene. In contrast, IL-2 decreased transcription from the IL-2 receptor gamma chain promoter. These results suggest that expression of the gamma chain is regulated at the transcription level by extracellular stimuli and may be implicated in immune response.

Transcriptional regulation of the elastin gene by insulin-like growth factor-I involves disruption of Sp1 binding. Evidence for the role of Rb in mediating Sp1 binding in aortic smooth muscle cells

We have recently identified a novel element (EFE 5/6) in the human elastin gene promoter that modulates the ability of insulin-like growth factor I (IGF-I) to up-regulate elastin gene transcription in aortic smooth muscle cells. In the present study, we have pursued the identification of those nuclear proteins binding to the EFE 5/6 element and affected by IGF-I treatment. Chelation inactivation and metal reactivation experiments together with supershift gel analyses demonstrated that Sp1 was one of the proteins affected by IGF-I. Southwestern and Western analyses showed that Sp1 was present in IGF-I nuclear extracts and capable of binding DNA after fractionation. Addition of retinoblastoma gene product (Rb) antibody mimicked the effect of IGF-I in gel shift analysis, suggesting that Sp1 binding may be regulated by an inhibitor normally associated with Rb. The fact that the phosphorylation state of Rb was affected by IGF-I was shown by Western blot analysis. The control smooth muscle cells transcribed the elastin gene at a high level without addition of IGF-I, so it is likely that disruption of Sp1 binding is the first step in allowing the binding of a more potent activating factor.

Epidermal growth factor stimulation of the human gastrin promoter requires Sp1

Growth factors coordinately regulate a variety of different genes to stimulate cellular proliferation. In the stomach, gastrin, epidermal growth factor (EGF), and transforming growth factor-alpha all mediate gastric mucosal homeostasis by promoting cell renewal. We have previously shown that EGF and phorbol esters stimulate the human gastrin promoter through a novel GC-rich DNA element 5'-(68)GGGGCGGGGTGGGGGG-53 called gERE (gastrin EGF response element). In this report, we show that three factors bind to this element, the transcription factor Sp1 and two fast migrating complexes designated gastrin EGF response proteins (gERP 1 and 2). To understand how these factors bind and confer EGF responsiveness, mutations of gERE were tested in vitro for protein binding and in vivo for promoter activation. Both gel shift assays and UV cross-linking studies revealed that the factors bind to overlapping domains, Sp1 to the 5' half-site and gERP 1 and 2 to the 3' half-site. Placing either the 5' or 3' mutations upstream of a minimal gastrin promoter abolished EGF induction. Therefore both the 5' and 3' domains were required to confer EGF induction. Collectively, these results demonstrate that complex interactions between Sp1 and other factors binding to overlapping gERE half-sites confer EGF responsiveness to the gastrin promoter.

Regulation of rat ornithine decarboxylase promoter activity by binding of transcription factor Sp1

Ornithine decarboxylase (ODC) is the rate-limiting enzyme of polyamine biosynthesis. We investigated the transcriptional regulation of the rat ODC gene using transient expression assays. The 5'-flanking region (-1156 to +13) of the ODC gene was sufficient to mediate strong basal expression of a luciferase reporter. Sequences between -345 and -93 contributed to basal promoter activity. This region, containing five potential Sp1 binding sites, was analyzed by electrophoretic mobility shift assays. Three specific DNA-protein complexes were identified using H35 nuclear extracts and the -345/-93 ODC probe. Binding to all three was eliminated by competition with an oligonucleotide containing an Sp1 binding site, but not by a mutant Sp1 oligonucleotide. Preincubation with an antibody against Sp1 supershifted complexes associated with one or more of Sp1 binding sites 1-4 as well as with site 5. DNase I footprinting revealed two protected regions: PR-I (-92 to -130) and PR-II (-304 to -332). PR-I contains a putative binding site for Sp1 that was protected by recombinant Sp1 protein. Transfection studies in Schneider SL2 cells demonstrated that the ODC promoter is transactivated up to 350-fold by Sp1 and that this transactivation is dependent on the presence of Sp1 binding sites 1-4. Thus, although the ODC promoter binds multiple nuclear proteins, Sp1 or a related protein appears to be a critical determinant of ODC transcription, possibly through cooperative interactions between Sp1 and additional transcription factors.

Multiple upstream promoter elements of the gene for the pregnancy-associated tartrate-resistant acid phosphatase, uteroferrin bind human endometrial nuclear proteins

The endometrial expression of the gene encoding porcine uteroferrin (UF), during pregnancy is presumed to be mediated by cis-regulatory regions distinct from those that confer its limited expression to other mammalian tissues and cell types. In the present study, chimeric DNA constructs of native and progressive 5' deleted promoter regions fused to the promoter chloramphenicol acetyl-transferase reporter gene were transiently transfected in the human endometrial carcinoma cell line ECC-1 to examine their ability to direct UF promoter activity. The region between -1935 and -831 bp contained negatively acting elements which drastically reduced basal promoter activity. In contrast, the region between -831 and -484 bp contributed significantly to high level basal activity. Gel retardation and footprinting assays identified factor-binding sites between -1601 and -484 bp for human endometrial nuclear proteins. One binding site corresponds to a heptamer motif (TGCTAGA) present twice within the -1601 to -831 bp region and previously shown to bind an 80 kDa porcine endometrial protein. This heptamer bound an 80 kDa nuclear protein from human ECC-1 and human Ishikawa endometrial cells and a 92 kDa protein from human placental JEG-3 cells. The other binding region within -831 to -484 bp contained GC-rich sequences, which bind human Sp1. The protected GC-rich sequence (GC-Box 1) between -768 and -749 bp also binds a 24 kDa M(r) protein. Nuclear proteins of molecular weight 40-60 kDa and distinct from Sp1, Sp2 and Sp3 bound a second GC-rich sequence (GC-Box 3) between -628 and -616 bp. These studies demonstrate that multiple elements within the UF gene promoter bind nuclear proteins which are similarly expressed in other endometrial cells and suggest that common transactivating factors may functionally mediate expression of endometrial-associated genes.

Regulation of the tissue factor promoter in endothelial cells. Binding of NF kappa B-, AP-1-, and Sp1-like transcription factors

Tissue factor is up-regulated on endothelial cells and monocytes in response to cytokines and endotoxin and is the main trigger of the extrinsic pathway of the coagulation cascade. We have isolated the porcine tissue factor gene and studied the regulation of the promoter, which has not been investigated previously in endothelial cells. Comparison of the promoter sequences with the respective human and murine genes reveals short stretches of homology, which encompass potential binding sites for AP-1, NF kappa B, and Sp1 transcription factors. Using DNase I footprinting, we detect binding of nuclear factors to these promoter elements. Transfection experiments demonstrate that a 300-base pair fragment containing the conserved elements can mediate induced transcription and that the NF kappa B-like element is essential. In accordance, electrophoretic mobility shift assays show a strong increase in the binding of factors to the NF kappa B-like site following induction. We further provide evidence that RelA (p65), c-Rel, and possibly novel polypeptides bind to the tissue factor NF kappa B element. In addition, we show constitutive binding of members of the Fos/Jun and Sp1 families to the AP-1 and Sp1 sites, respectively. We propose a concerted action of AP-1-, NF kappa B-, and Sp1-like factors in transcription from the tissue factor promoter in endothelial cells.

GATA-3 dominant negative mutant. Functional redundancy of the T cell receptor alpha and beta enhancers

The GATA family of transcription factors regulates a wide variety of genes, including those involved in differentiation of erythrocytes and T lymphocytes. We report here the creation of a dominant negative mutant of GATA-3, KRR, which effectively blocks wild-type GATA-1, GATA-2, and GATA-3 transactivation when co-expressed in transient assays. KRR was generated by site-directed mutagenesis while investigating a putative activation domain of GATA-3, located between its two zinc fingers. The GATA-3 KRR mutation does not affect expression, nuclear translocation, or the ability to bind to a consensus GATA sequence. KRR can suppress the activity of the minimal T cell receptor (TCR) alpha and beta enhancers by 12- and 3.4-fold, respectively. However, KRR did not have a significant effect on the activity of larger TCR-alpha and -beta enhancer fragments. Thus, functional redundancy in the TCR-alpha and -beta enhancers can compensate for the loss of GATA-3 activity.

Major histocompatibility class I molecules can present cryptic translation products to T-cells

Self or foreign cellular proteins provide peptides for presentation by major histocompatibility complex (MHC) class I molecules on the surface of antigen presenting cells (APC). Surprisingly, several studies have shown that T-cells can recognize APC transfected with antigen genes that were not present in the appropriate translational context. To understand the basis of this phenomenon, APC were transfected with DNA constructs encoding the OVA257-264 (SL8) peptide, but with varying translation initiation codons. We report that, in addition to ATG, 6 other codons (ATT, ACG, CTG, GCG, TGG, GAT) also allowed presentation to SL8-Kb-specific T-cells. Significantly, this set includes 3 of 4 known non-ATG translation initiation codons strongly suggesting that cryptic translation accounts for this phenomenon. Although expression of the SL8-Kb complex was readily detected by T-cell activation, the amount of processed peptides was below detection limit (< 30 copies/cell) in cell extracts. Thus, the fortuitous presence of these cryptic translation initiation sites in transcribed genes can explain how peptide MHC complexes were obtained in sufficient amounts for T-cell activation. The translation initiation codons identified here could also be useful for identifying potential open reading frames that possess biological and/or immunological activities.

Cooperation by sterol regulatory element-binding protein and Sp1 in sterol regulation of low density lipoprotein receptor gene

Regulation of the low density lipoprotein (LDL) receptor promoter by cholesterol requires a well defined sterol regulatory site and an adjacent binding site for the universal transcription factor Sp1. These elements are located in repeats 2 and 3 of the wild type promoter, respectively. The experiments reported here demonstrate that Sp1 participates in sterol regulation of the LDL receptor in an orientation-specific fashion. We present data which suggest that sterol regulatory element-binding protein (SREBP) increases the binding of Sp1 to the adjacent repeat 3 sequence. We also demonstrate that SREBP and Sp1 synergistically activate expression from the LDL receptor promoter inside the cell by cotransfecting expression vectors encoding each protein into Drosophila tissue culture cells that are devoid of endogenous Sp1. In addition, other transcription factor sites were unable to substitute for Sp1 in sterol regulation when placed next to the SREBP-binding site. These studies together with recent data from others provide the basis of a working model for sterol regulation of the LDL receptor promoter. The presence of Sp1 sites in several other regulated promoters suggests that this universal transcription factor has been recruited to participate in many regulatory responses possibly by a similar mechanism.

Promoter function of the human glucose-6-phosphate dehydrogenase gene depends on two GC boxes that are cell specifically controlled

Human glucose-6-phosphate dehydrogenase is expressed in all cells by a housekeeping gene whose regulatory 5'-flanking sequence includes at least nine GC boxes. By transient transfection of HeLa and HepG2 cells with constructs containing glucose-6-phosphate dehydrogenase gene regions linked to a reporter gene, we have now delineated the core promoter and have located upstream stimulatory and inhibitory sequences. By mutational analysis, we demonstrate that the activity of the core promoter requires two out of seven GC boxes. We show that stimulatory protein 1 (Sp1)-related factors and activator protein 2 (AP-2)-related proteins bind to these two boxes in band-shift experiments. One point mutation that affects the binding of only the Sp1-related factors to one or both boxes causes a marked decrease of promoter activity in HepG2 cells but not in HeLa cells. We conclude that (a) two out of many seemingly redundant GC boxes are necessary to drive a G+C-rich housekeeping promoter; (b) factors that bind to GC boxes may exert cell-type-specific regulation of housekeeping gene promoter activity; (c) point mutations in the promoter of the glucose-6-phosphate dehydrogenase gene can inhibit its transcription.

Triplex formation at the rat neu oncogene promoter

Current cancer chemotherapy treatments generally act by affecting rapidly growing malignant cells. Unfortunately, they are relatively nonspecific and thus have a tendency to affect other rapidly growing normal cells in a deleterious manner. Triplex-forming oligodeoxyribonucleotides (TFOs) promise to be a new class of sequence-specific DNA-binding drugs which will target malignancies at the transcriptional level. The formation of an intermolecular triplex (triple helix) has been shown to block the binding of transcription factors and repress transcription in genes such as c-myc and that encoding the epidermal growth factor receptor. The rat neu oncogene promoter contain promoter-enhancer elements which are purine/pyrimidine rich. These enhancer elements are amenable to targeting by TFOs. the human counterpart of rat neu, HER2, is often found to be amplified or overexpressed in a variety of malignancies, such as those of the breast, lungs, ovary, colon and stomach. TFOs may proved to be the basis of effective chemotherapy drugs for these cancers. TFO binding at the "GTG" element (5'GGTGGGGGGG) and at the 'GA' element (5'GGAGGAGGAGGG) has been characterized by gel mobility shift analysis and DNase 1 footprinting. Binding has been shown to occur at a Kd as low as 10(-8) M and has been shown to be sequence specific.

Multiple regulatory elements control the basal promoter activity of the human alpha 4 integrin gene

It has been suggested that expression of the genes encoding the alpha 4/beta 1 integrin increases during wound healing of the cornea. As a first step in understanding the mechanisms required to stimulate alpha 4 gene expression during this process, we defined the minimal upstream sequence required to direct basal promoter activity for this gene. Using deletion analyses of the alpha 4 gene upstream sequence, we identified two functionally important negative regulatory elements. Dimethylsulfate (DMS) methylation interference assays provided evidence for the binding of a single nuclear protein to tandemly repeated homologous cis-acting elements (designated alpha 4.1 and alpha 4.2) from the alpha 4 basal promoter that share the core sequence 5'-GTGGGT-3'. The formation of a protection only at alpha 4.1 in DNase I footprinting suggested that it is the primary target element for the binding of nuclear proteins. Three distinct nuclear proteins bound a double-stranded oligonucleotide bearing the DNA sequence of alpha 4.1 to produce specific DNA-protein complexes (R1 to R3) in gel-shift assays, from which that producing R3 was identified as the protein yielding DNase I protection at alpha 4.1. Detailed mutational analysis of alpha 4.1 and alpha 4.2 indicated that both elements positively regulate gene expression in primary cultures of corneal epithelial cells and Jurkat tissue culture cells, which is consistent with the deletion analysis. However, when transiently transfected into pituitary GH4C1, the alpha 4.2 mutants yielded increased chloramphenicol acetyl transferase activity therefore demonstrating that these elements have the ability to function either as positive or negative regulators of gene transcription in a manner that is dependent on the type of cell transfected.

Identification of human T-cell lymphotropic virus type I 21-base-pair repeat-specific and glial cell-specific DNA-protein complexes

The human T-cell lymphotropic virus type I (HTLV-I)-encoded protein, Tax, is capable of trans-activating HTLV-I transcription by interacting with specific sequences in the HTLV-I long terminal repeat (LTR) which comprise an inducible enhancer containing three imperfect tandem repeats of a 21-bp sequence. There is no evidence that purified Tax can bind to DNA in the absence of cellular factors, suggesting that Tax most likely regulates transcription via interaction with cellular factors. Since HTLV-I is a documented agent of adult T-cell leukemia and tropical spastic paraparesis, disorders of the immune and nervous systems, respectively, characterization of cellular factors of lymphoid and neuroglial origin which interact with the 21-bp repeat elements is essential to understanding of the mechanisms involved in basal and Tax-mediated transcription in cells of immune and nervous system origin. Utilizing electrophoretic mobility shift (EMS) analyses, we have detected both 21-bp repeat-specific and glial cell-specific DNA-protein complexes. Several 21-bp repeat-specific DNA-protein complexes were detected when nuclear extracts derived from cells of lymphoid (Jurkat, SupT1, and H9), neuronal (IMR-32 and SK-N-MC), and glial (U-373 MG, Hs683, and U-118) origin were used in reactions with each of the three 21-bp repeat elements. In addition, a glial cell-specific DNA-protein complex was detected when nuclear extracts derived from U-373 MG, Hs683, and U-118 glial cell lines reacted with the promoter-distal and central 21-bp repeat elements. Furthermore, EMS analyses performed with nuclear extracts derived from lymphocytic and glial cell origin and a 223-bp fragment of the HTLV-I long terminal repeat encompassing the three 21-bp repeat elements (designated Tax-responsive elements 1 and 2, TRE-1/-2) have also resulted in the detection of glial cell type-specific DNA-protein complexes. Competition EMS analyses with oligonucleotides containing transcription factor binding site sequences indicate the involvement of a cyclic AMP response element binding protein in the formation of DNA-protein complexes which form with all three 21-bp repeat elements and the glial cell-specific DNA-protein complex as well as the involvement of Sp1 or an Sp1-related factor in the formation of the 21-bp repeat III-specific DNA-protein complexes.

Perspectives on zinc finger protein function and evolution--an update

Complexity is one of the hallmarks that applies to C2H2 type zinc finger proteins (ZFPs). Structurally distinct clusters of zinc finger modules define an extremely large superfamily of nucleic acid binding proteins with several hundred, perhaps thousands of different members in vertebrates. Recent discoveries have provided new insights into the biochemistry of RNA and DNA recognition, into ZFP evolution and genomic organization, and also into basic aspects of their biological function. However, as much as we have learned, other fundamental questions about ZFP function remain highly enigmatic. This essay is meant to define what we personally feel are important questions, rather than trying to provide a comprehensive, encyclopaedic review.

A locus control region in the T cell receptor alpha/delta locus

A locus control region (LCR) is a set of cis-acting elements that regulate chromatin accessibility of a gene locus. In the T cell receptor (TCR) alpha/delta locus, an LCR might regulate the differential tissue and developmental expression and the rearrangement of TCR alpha and delta genes. We have localized a region 3' of the TCR alpha/delta locus containing eight T cell-specific nuclease hypersensitive domains (HS-1 to HS-8), which fit the characteristics of an LCR. In transgenic mice, a TCR alpha gene linked to this region is expressed at a high level, independent of the site of integration and correlates with gene copy number. The transgene is expressed in the alpha beta but not the gamma delta T cell subset and is activated at the right time during development. Proper LCR function requires the region containing HS-2 to HS-6. We propose a model of LCR competition to explain the differential regulation of TCR alpha versus delta genes during development.

Regulation of transcription from the human muscle phosphofructokinase P2 promoter by the Sp1 transcription factor

The human muscle phosphofructokinase (HPFKM) p2 promoter contains sequence elements that are similar to the Sp1 transcription factor binding site consensus sequence. DNase I footprinting identified four regions of the HPFKM p2 promoter that bound purified Sp1. Gel retardation analysis using HeLa S3 nuclear extracts and purified Sp1 protein demonstrated that each of the four recognition elements bound the Sp1 transcription factor. The function of the HPFKM p2 promoter elements was examined in transient transfection assays using these binding sites cloned into a minimal promoter element. In Drosophila Schneider line-2 cells, each of these regulatory regions trans-activated transcription from a minimal promoter element in response to exogenously expressed Sp1. In addition, transcription from the HPFKM p2 promoter was shown to be trans-activated by exogenously expressed Sp1 in Drosophila Schneider line-2 cells. Deletion analysis of the HPFKM p2 promoter demonstrated that the promoter region between -66 and +16 was sufficient to confer sp1 responsiveness. This promoter region includes one of the regulatory elements footprinted by the purified Sp1 transcription factor and mediates the majority of the transcriptional activity from the HPFKM p2 promoter in the human cervical carcinoma cell line HeLa S3. This demonstrates that the HPFKM p2 promoter contains four functional Sp1 binding sites that may contribute to the level of transcription from this promoter in a variety of cell types.

Tissue-specific differences in the expression of the human ADH2 alcohol dehydrogenase gene and in binding of factors to cis-acting elements in its promoter

The human alcohol dehydrogenase gene ADH2 is expressed at high levels in liver, at lower levels in kidney and several other tissues, and is not expressed in other tissues such as spleen. This pattern of expression suggests a complex regulatory region that responds to a variety of transcription factors in different cellular contexts. Seven cis-acting sequences in the proximal 271 bp of the ADH2 promoter were mapped. The occupancy of these sites differed markedly among extracts from liver, kidney, spleen, H4IIE-C3 cells, HeLa cells, and CV-1 cells. These differences in occupancy were accompanied by differences in gene expression in the three cell lines. The ADH2 promoter directed substantial CAT expression in H4IIE-C3 cells (rat hepatoma) and in HeLa cells, but only minimal expression in CV-1 cells (monkey kidney fibroblasts). The three cell lines differed in the effects of deletions within the promoter. An ADH2 promoter that contained both the USF/MLTF site and the G3T site gave four- to eight-fold higher expression in both H4IIE-C3 and HeLa cells than a smaller promoter that lacked these sites; in contrast, these sequences did not significantly stimulate transcription in CV-1 cells. A CTF/NF-I-related site acted as a negative element in all three cell lines. Coexpression of C/EBP alpha altered the cell specificity. The ADH2 promoter was moderately stimulated (two-fold) by coexpression of C/EBP alpha in H4IIE-C3 cells, but markedly stimulated in HeLa cells and in CV-1 cells (11- and 20-fold, respectively). These results demonstrate the differential importance of cis-acting sequences and of specific transcription factors in different cells, which allows regulated expression of ADH2 in multiple tissues.

Requirement for the orphan steroid receptor Nur77 in apoptosis of T-cell hybridomas

Apoptosis is a phenomenon observed during development of many cell types in many organisms. It is an internal, programmed cell death characterized by DNA fragmentation into nucleosome-size pieces. Anti-CD3-induced apoptosis in T-cell hybridomas and immature thymocytes requires new gene transcription and may be related to negative selection during T-cell development. Using subtractive hybridization, we isolated a complementary DNA clone encoding the orphan steroid receptor Nur77 (refs 7-9). It shows different patterns of messenger RNA induction between apoptotic and stimulated T cells. We report here the use of gel shift analysis to demonstrate that the Nur77 protein is present at high levels in apoptotic T-cell hybridomas and apoptotic thymocytes, but not in growing T cells or stimulated splenocytes. A Nur77 dominant negative protected T-cell hybridomas from activation-induced apoptosis. Hence Nur77 is necessary for induced apoptosis in T-cell hybridomas.