Maximal serum stimulation of the c-fos serum response element requires both the serum response factor and a novel binding factor, SRE-binding protein

Molecular epidemiology of avian leukosis virus subgroup J and evolutionary history of its 3' untranslated region

Avian leukosis subgroup J (ALV-J) causes a variety of tumors and mortality in meat-type chickens. Since its discovery in the late 1980s, ALV-J has spread to breeding stock produced by most primary breeding companies of North America, the European Union, and Asia. ALV-J seems to have been eradicated from elite breeding stock produced by most primary breeders, albeit ALV-J still circulates in some commercial poultry. This study was undertaken to examine the molecular epidemiology and evolution of ALV-J detected in breeding stock and broiler chickens representing eight primary breeding companies over a period of approximately 20 yr (1988-2007). The redundant transmembrane region of the envelope gene has been deleted in some isolates, suggesting that this region is dispensable for viral fitness. Within the 3' untranslated region (3' UTR), the direct repeat 1 was present in 100% of the ALV-J isolates studied. In contrast, the E element has undergone substantial deletions in >50% of the ALV-J proviruses studied. Overall, the unique region 3 was the least conserved within the 3' long terminal repeat (LTR), albeit the transcriptional regulatory elements typical of avian retroviruses (CAAT, CArG, PRE, TATA, and Y boxes) were highly conserved. The direct repeat region of the LTR was identical in all of the proviruses, and the 3' unique region 5 was relatively well conserved. Thus, the 3' UTR of ALV-J has evolved rapidly, reflecting significant instability of this region. Some of the mutations in the 3' UTR have resulted in the emergence of moderately distinct genetic lineages representing each primary breeding company from which ALV-J was isolated.

Sequestration of serum response factor in the hippocampus impairs long-term spatial memory

The formation of long-term memory has been shown to require protein kinase-mediated gene expression. One such kinase, mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK), can lead to the phosphorylation of serum response factor (SRF) and Elk-1, enhancing the expression of target genes. However, a direct involvement of these transcription factors in memory storage has not been demonstrated. We have employed an oligonucleotide decoy technique to interrogate SRF and Elk-1. Previously, it has been shown that intra-amygdalal infusion of small double-stranded decoy oligonucleotides for nuclear factor-kappaB (NFkappaB) can impair long-term memory for fear-potentiated startle. Using this approach, we found that intra-hippocampal infusion of NFkappaB decoy oligonucleotides also impairs long-term spatial memory, consistent with a role for this factor in long-term memory storage. Decoy oligonucleotides containing the binding site for SRF, as confirmed by shift-western, did not influence memory acquisition but impaired long-term spatial memory. Analysis of search behavior during the transfer test revealed deficits consistent with a loss of precise platform location information. In contrast, oligonucleotides with a binding site for either Elk-1 or another target of ERK activity, SMAD3/SMAD4, did not interfere with memory formation or storage. These findings suggest that SRF-mediated gene expression is required for long-term spatial memory.

The RIPE3b1 activator of the insulin gene is composed of a protein(s) of approximately 43 kDa, whose DNA binding activity is inhibited by protein phosphatase treatment

Glucose-stimulated and pancreatic islet beta cell-specific expression of the insulin gene is mediated in part by the C1 DNA-element binding complex, termed RIPE3b1. In this report, we define the molecular weight range of the protein(s) that compose this beta cell-enriched activator complex and show that protein phosphatase treatment inhibits RIPE3b1 DNA binding activity. Fractionation of beta cell nuclear extracts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that RIPE3b1 binding was mediated by a protein(s) within the 37-49-kDa ranges. Direct analysis of the proteins within the RIPE3b1 complex by ultraviolet light cross-linking analysis identified three binding species of approximately 51, 45, and 38 kDa. Incubating beta cell nuclear extracts with either calf alkaline phosphatase or a rat brain phosphatase preparation dramatically reduced RIPE3b1 DNA complex formation. Phosphatase inhibition of RIPE3b1 binding was prevented by sodium pyrophosphate, a general phosphatase inhibitor. We discuss how changes in the phosphorylation status of the RIPE3b1 activator may influence its DNA binding activity.

Four isoforms of serum response factor that increase or inhibit smooth-muscle-specific promoter activity

Serum response factor (SRF) is a key transcriptional activator of the c-fos gene and of muscle-specific gene expression. We have identified four forms of the SRF coding sequence, SRF-L (the previously identified form), SRF-M, SRF-S and SRF-I, that are produced by alternative splicing. The new forms of SRF lack regions of the C-terminal transactivation domain by splicing out of exon 5 (SRF-M), exons 4 and 5 (SRF-S) and exons 3, 4 and 5 (SRF-I). SRF-M is expressed at similar levels to SRF-L in differentiated vascular smooth-muscle cells and skeletal-muscle cells, whereas SRF-L is the predominant form in many other tissues. SRF-S expression is restricted to vascular smooth muscle and SRF-I expression is restricted to the embryo. Transfection of SRF-L and SRF-M into C(2)C(12) cells showed that both forms are transactivators of the promoter of the smooth-muscle-specific gene SM22alpha, whereas SRF-I acted as a dominant negative form of SRF.

Sequences at the 3' side of the c-fos SRE mediate gene expression via an Sob1-dependent, TCF-independent pathway

We previously described a 110-kDa tyrosine phosphoprotein, Sob 1, that regulates formation of the DNA binding complex Band A at the c-fos serum response element (SRE) during T cell activation. Using competition and mutant oligonucleotide analysis, we have determined that both the core CArG box of the c-fos SRE and the 3' sequences flanking the CArG box are necessary for stable Band A complex formation. Moreover, using transient transfection and reporter assays, we show that mutations affecting Band A complex formation in vitro also impaired serum induction of c-fos gene expression in vivo. Since mutation at this site has no effect on SRF binding, our results suggest that in combination with SRE/SRF, Sob 1-regulated factor(s) bind at the 3' side of SRE to form Band A, and this confers maximal serum induction of c-fos gene expression via the SRE.

Ras regulates the association of serum response factor and CCAAT/enhancer-binding protein beta

The serum response element (SRE) is a promoter element essential for transcriptional activation of immediate early genes, such as c-fos and early growth response-1, by mitogenic signals. Several transcription factors bind the SRE, including the serum response factor (SRF), the ternary complex factor, and the CCAAT/enhancer-binding protein beta (C/EBPbeta). The C/EBPbeta mRNA encodes three translation products of 38, 35, and 20 kDa. p35-C/EBPbeta activates transcription of the SRE in an SRF-dependent fashion, whereas p20-C/EBPbeta, which initiates at an internal in-frame methionine, lacks a transactivation domain and inhibits transcription. We show that SRF and C/EBPbeta interact in vivo through the DNA binding domain of SRF and the C terminus of C/EBPbeta common to p35/38 and p20. Therefore, like the ternary complex factor, C/EBPbeta may be recruited to the SRE not only by binding to the DNA, which is not a high affinity site, but also by protein-protein interactions with SRF. Strikingly, in both the mammalian two-hybrid assay and in vivo coimmunoprecipitations, the association of SRF and p35-C/EBPbeta but not p20-C/EBPbeta is dramatically stimulated by activated Ras. Furthermore, mutation of the threonine within a mitogen-activated protein kinase consensus motif in the C terminus of C/EBPbeta eliminates the response to Ras. These results suggest a new mechanism by which mitogenic signals may influence transcription activity of the SRE by selectively promoting protein-protein interactions between SRF and the transactivator p35-C/EBPbeta.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Enhanced serum response element binding activity correlates with down-regulation of c-fos mRNA expression in the rat brain following repeated cortical lesions

Repeated lesions of rat cerebral cortex result in transient peaks in the level of the c-fos transcript, but after the second lesion, this peak is substantially diminished. Using this lesion paradigm, we have analyzed the participation of the c-fos promoter elements SRE and DSE in the regulation of c-fos transcription. Following a single lesion, SRE/DSE binding activity peaked at 2 h, subsequent to the maximal levels of c-fos mRNA and parallel to the peak of c-Fos protein. After a second lesion (reinduction), 4 h following the initial lesion, SRE/DSE binding activity peaked after only 30 min and was significantly higher than following the first lesion. Once again, this peak occurred after the peak of c-fos mRNA expression and parallel with the second peak of c-Fos protein expression. These results suggested that the SRE and DSE promoter elements participated in the induction and down-regulation of c-fos transcription in vivo and suggested the possible involvement of Fos protein in its own regulation. The ability of Fos/Fra proteins to participate in a transcriptional complex was confirmed in gel-shift experiments with an AP-1 element, and the biphasic trend of binding activity was observed. Supershift experiments were performed to directly determine whether Fos protein was participating in SRE and/or DSE transcriptional complexes. No alterations in the position or intensity of the shifted band were observed using Fos/Fra antiserum suggesting that Fos/Fra proteins could be involved in c-fos down-regulation through mechanisms other than direct participation in the SRE/DSE transcription complex.

Expression of the serum response factor gene is regulated by serum response factor binding sites

The serum response factor (SRF) is a ubiquitous transcription factor that plays a central role in the transcriptional response of mammalian cells to a variety of extracellular signals. Notably, SRF has been found to be a key regulator of members of a class of cellular response genes termed immediate-early genes (IEGs), many of which are believed to be involved in regulating cell growth and differentiation. The mechanism by which SRF activates transcription of IEGs in response to mitogenic agents has been extensively studied. Significantly less is known about how expression of the SRF gene itself is mediated. We and others have previously shown that the SRF gene is itself transiently induced by a variety of mitogenic agents and belongs to a class of "delayed" early response genes. We have cloned the SRF promoter and in the present study have analyzed the upstream regulatory sequences involved in mediating serum responsiveness of the SRF gene. Our analysis indicates that inducible SRF expression requires both SRF binding sites located within the first 63 nucleotides upstream from the start site of transcriptional initiation and an Sp1 site located 83 nucleotides upstream from the start site. Maximal transcriptional activity of the promoter also requires two CCAATT box sites located 90 and 123 nucleotides upstream of the start site.

Unique long terminal repeat U3 sequences distinguish exogenous jaagsiekte sheep retroviruses associated with ovine pulmonary carcinoma from endogenous loci in the sheep genome

Ovine pulmonary carcinoma (OPC) is a contagious lung cancer of sheep that is presumed to be caused by an exogenous retrovirus of sheep, jaagsiekte sheep retrovirus (JSRV). The sheep genome carries 15 to 20 copies of endogenous sheep retrovirus (ESRV) loci that hybridize to JSRV DNA probes. In order to clarity the etiologic roles of ESRV and an exogenous JSRV-like retrovirus (exJSRV) in OPC, we assessed sequence differences between ESRV and JSRV. Molecular characterization of six ESRV loci revealed restriction sites specific for JSRV. Nucleotide sequences of ESRVs from sheep of different breeds were similar to those of JSRV in structural genes but divergent in U3. Therefore, primers specific for the U3 sequences of exJSRV were designed for use in the PCR. Of 13 tumor DNAs tested by PCR with these exogenous-virus U3 primers, 8 produced DNA fragments that hybridized with the JSRV gag probe, but neither lung DNAs from healthy sheep nor DNAs from nontumor tissues of diseased sheep produced similar DNA fragments. exJSRV PCR products from tumor DNAs of sheep with OPC from three continents had restriction profiles similar to each other but different from those of ESRVs upon digestion with EcoRI, HindIII, NdeI, KpnI, and ScaI. These exjSRVs could be classified into two genotypes according to U3 sequences and restriction profiles. U3 sequences of exJSRV proviruses in tumors strongly resembled those of JSRV but differed from those of ESRVs, suggesting that exJSRVs, rather than ESRVs, are primarily associated with oncogenesis in OPC.

Epidermal growth factor induces H+,K+-ATPase alpha-subunit gene expression through an element homologous to the 3' half-site of the c-fos serum response element

Epidermal growth factor (EGF) acutely inhibits acid secretion; however, prolonged administration of EGF has been reported to increase acid production. We undertook these studies to examine whether the physiological effects of EGF on acid secretion are mediated by regulation of gastric H+,K+-ATPase, the principle enzyme responsible for acid secretion. EGF in concentrations equivalent to those in plasma increased H+,K(+)-ATPase alpha-subunit mRNA levels. Using H+,K(+)-ATPase-luciferase constructs transfected into primary cultured parietal cells, a significant step up in EGF inducibility was observed between bases -162 and -156 (5'-GACATGG-3') relative to the cap site. This EGF response element (ERE) conferred EGF inducibility when linked to homologous and heterologous promoters. The ERE is homologous to the 3' half-site of the c-fos serum response element to which rNFIL-6, rE12, and SRE-ZBP bind. Electrophoretic mobility shift assays using an ERE probe and parietal cell nuclear extracts revealed a specific DNA-protein complex, the formation of which was changed by neither E12 and NFIL-6 consensus oligonucleotides nor antibodies for NFIL-6, SRE-ZBP, and E12. Our studies indicate that EGF induces gastric H+,K(+)-ATPase alpha-subunit gene expression via an interaction between a specific ERE and a novel transcriptional factor and that this may be a physiologic mechanism by which EGF regulates acid secretion.

Induction of tissue inhibitor and matrix metalloproteinase by serum in human heart-derived fibroblast and endomyocardial endothelial cells

To understand the regulatory mechanisms of extracellular matrix (ECM) turnover and proteinase expression in human cardiovascular tissue, we have isolated and characterized human heart fibroblast (HHF) and human heart endothelial (HHE) cells from endomyocardial biopsy specimens. HHE cell in culture exhibited the typical cobblestone growth pattern and positive immunofluorescent staining for factor VIII related antigen. HHF demonstrated the typical spindle shape during culture and were positive for vimentin. Both cell types were negative for alpha-actin, indicating that these cells were of nonmuscle origin. Cell growth studies revealed significant growth when maintained in limiting serum concentration, suggesting mitogenic activity of these cells, and demonstrated growth inhibitory activity when grown in serum-free medium. Serum-dependent matrix metalloproteinases (MMPs) and tissue inhibitor of metalloproteinases (TIMPs) expression was measured by zymography, immunoblot, and Northern blot analysis. Results indicated that serum induces both the MMP and TIMP expression at the mRNA and protein levels in a dose-dependent manner. This induction was inhibited by actinomycin D and cycloheximide, suggesting transcriptional and translational regulation of MMP and TIMP. Indirect immunofluorescence labeling indicated expression of MMP and TIMP in HHF and HHE cells. These results suggested that the serum induces proliferation as well as expression of MMP and TIMP in HHE and HHF cells. The growth inhibitory activity of these cell cultures will enable us to explore further the nature of this response and compare this phenomenon with other growth inhibitors and growth promoters identified in other normal and transformed cells.

Identification of a new enhancer in the promoter region of human TR3 orphan receptor gene. A member of steroid receptor superfamily

Human TR3 orphan receptor is a member of the steroid/thyroid hormone receptor superfamily and is the human homologue of the proteins encoded by the rat NGFI-B and mouse nur77 genes. These genes are induced rapidly by androgens/growth factors and may have functions related to cell proliferation, differentiation, and apoptosis. To investigate the TR3 orphan receptor gene transcriptional regulation, a 2.3-kilobase genomic DNA fragment containing the TR3 orphan receptor gene promoter region was isolated, sequenced, and characterized. Sequence homology search within this promoter region revealed some potential cis-acting elements such as cAMP response element, interleukin-6 response element, estrogen response element, and GC box. Deletion analysis and chloramphenicol acetyltransferase assay also showed a novel cis-acting element of TR3 orphan receptor gene (NCAE-TR3), 200-181 base pairs upstream of the transcriptional start site. Gel retardation assay further demonstrated that some nuclear factors can bind to this NCAE-TR3. Together, our data suggest that NCAE-TR3 could be a new enhancer element associated with the transcription of an early response gene for mitogenesis and apoptosis.

Serum response element associated transcription factors in mouse embryos: serum response factor, YY1, and PEA3 factor

Many mammalian transcription factors, including human and mouse serum response factors (SRFs), are post-translationally modified with O-linked N-acetylglucosamine monosaccharides on multiple serine and/or threonine residues. Nuclear extracts were prepared from 9.5 to 19 days postcoitum mouse embryos and subsequently were fractionated by wheat germ agglutinin (WGA)-agarose affinity chromatography. SRF binds WGA-agarose and apparently is O-glycosylated. On the other hand, the low molecular weight serum response element (SRE)-binding proteins, including the previously named band I and band II factors, did not bind WGA-agarose. Furthermore, we showed that the fastest migrating complex contains the Yin-Yang 1 (YY1) factor. YY1 binds to the c-fos SRE and skeletal alpha-actin muscle regulatory element (MRE), but not the cardiac alpha-actin MRE. Nuclear extracts from NIH/3T3 fibroblasts contain similar, if not identical, SRE-binding complexes. Besides these SRE-binding factors, mouse PEA3-binding factor, presumably an ETS domain-containing protein, was found to bind SRF protein. This physical interaction, between SRF and ETS domain proteins, was shown to involve the DNA-binding domain-containing region of SRF and not the carboxyl-terminal transactivation domain.

Mediation of suppression of c-fos transcription in rasT24-transformed rat cells by a cis-acting repressor element

Prolonged expression of activated ras mutants resulted in both neoplastic transformation and suppression of serum-induced c-fos expression in Rat1 fibroblasts. Expression of other serum-inducible genes, including c-jun and beta-actin, was not suppressed in ras-transformed Rat1 cells, indicating that these effects are specific for c-fos and that growth-factor signal transduction pathways remain essentially intact. Run-on transcription studies indicated that c-fos transcription was blocked at the level of initiation in these cells. Transient transfection studies using 360 bp from the wild-type c-fos promoter as well as a series of mutated c-fos promoter fragments linked to the chloramphenicol acetyltransferase gene indicated that repression of c-fos was mediated by approximately 49 bp immediately upstream of the dyad symmetry element (DSE). Deletion of this region, referred to as the upstream repressor region (URR), restored serum inducibility to the c-fos promoter in ras-transformed cells. In contrast, suppression of c-fos transcription was not affected by either deletion of 240 bp between the DSE and the TATA element or by base-substitution mutations that inactive the ternary complex factor and fos-AP-1-like binding sites. In addition, in vitro competition studies indicated that ras-transformed cells express one or more repressor factors that interact with as-yet-unidentified elements within the c-fos promoter (possibly the URR) and block serum induction of c-fos. These findings suggest that prolonged expression of activated ras results in the activation of one or more as-yet-unidentified proteins that suppress transcription of the c-fos gene by interacting with the URR.

Negative regulation of transcription in eukaryotes

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Characterization of nuclear proteins that bind the EFII enhancer sequence in the Rous sarcoma virus long terminal repeat

The EFII cis element is a 38-bp sequence at the 5' end of the Rous sarcoma virus long terminal repeat, extending from nucleotides -229 to -192 (with respect to the viral transcription start site), which is recognized by sequence-specific DNA-binding proteins in avian fibroblast nuclear extracts (L. Sealy and R. Chalkley, Mol. Cell. Biol. 7:787-798, 1987). We demonstrate that multiple copies of the EFII cis element strongly activate transcription of a reporter gene in vivo. We correlate the region of the EFII cis element which activates transcription in vivo with the in vitro binding site for three nuclear factors, EFIIa, EFIIb, and EFIIc. The sequence motif recognized by EFIIa, -b, and -c is also found in consensus binding sites for members of a rapidly growing family of transcription factors related to the CCAAT/enhancer-binding protein (C/EBP). EFIIa, -b, and -c are present in fibroblast and epithelial cell lines from various species but are much less abundant in differentiated rat liver and kidney cells. The EFIIa binding activity is particularly abundant in an avian B-cell lymphoma line. As judged from molecular weight analysis, cell type distribution, and sequence recognition properties, the EFII factors under study appear to differ from most of the previously described C/EBP-related factors and thus may expand the diversity of the C/EBP family.