Serum and v-src increase the level of a CCAAT-binding factor required for transcription from a retroviral long terminal repeat

NF-Y coassociates with FOS at promoters, enhancers, repetitive elements, and inactive chromatin regions, and is stereo-positioned with growth-controlling transcription factors

NF-Y, a trimeric transcription factor (TF) composed of two histone-like subunits (NF-YB and NF-YC) and a sequence-specific subunit (NF-YA), binds to the CCAAT motif, a common promoter element. Genome-wide mapping reveals 5000–15,000 NF-Y binding sites depending on the cell type, with the NF-YA and NF-YB subunits binding asymmetrically with respect to the CCAAT motif. Despite being characterized as a proximal promoter TF, only 25% of NF-Y sites map to promoters. A comparable number of NF-Y sites are located at enhancers, many of which are tissue specific, and nearly half of the NF-Y sites are in select subclasses of HERV LTR repeats. Unlike most TFs, NF-Y can access its target DNA motif in inactive (nonmodified) or polycomb-repressed chromatin domains. Unexpectedly, NF-Y extensively colocalizes with FOS in all genomic contexts, and this often occurs in the absence of JUN and the AP-1 motif. NF-Y also coassociates with a select cluster of growth-controlling and oncogenic TFs, consistent with the abundance of CCAAT motifs in the promoters of genes overexpressed in cancer. Interestingly, NF-Y and several growth-controlling TFs bind in a stereo-specific manner, suggesting a mechanism for cooperative action at promoters and enhancers. Our results indicate that NF-Y is not merely a commonly used proximal promoter TF, but rather performs a more diverse set of biological functions, many of which are likely to involve coassociation with FOS.

Recruitment of nuclear factor Y to the inverted CCAAT element (ICE) by c-Jun and E1A stimulates basal transcription of the bone sialoprotein gene in osteosarcoma cells

Bone sialoprotein (BSP), a major protein in the extracellular matrix of bone, is expressed almost exclusively by bone cells and by cancer cells that have a propensity to metastasize to bone. Previous studies have shown that v-src stimulates basal transcription of bsp in osteosarcoma (ROS 17/2.8) cells by targeting the inverted CCAAT element (ICE) in the proximal promoter. To identify possible downstream effectors of Src we studied the effects of the proto-oncogene c-jun, which functions downstream of Src, on basal transcription of bsp using transient transfection assays. Increased expression of endogenous c-Jun induced by the tumor promoter 12-O-tetradecanoyl-phorbol 13-acetate and ectopic expression of c-Jun increased basal transcription of chimeric reporter constructs encompassing the proximal promoter by 1.5-3-fold in ROS 17/2.8 osteosarcoma cells, with more modest effects in a normal bone cell line, RBMC-D8. The effects of c-Jun were abrogated by mutations in the ICE box and by co-expression of dominant negative nuclear factor Y, subunit A (NF-YA). The increase in bsp transcription did not require phosphorylation of c-Jun and was not altered by trichostatin treatment or by ectopic expression of p300/CREB-binding protein (CBP) or mutated forms lacking histone acetyltransferase (HAT) activity. Similarly, ectopic expression of p300/CBP-associated factor (P/CAF), which transduces p300/CBP effects, or of HAT-defective P/CAF did not influence the c-jun effects. Surprisingly, E1A, which competes with P/CAF binding to p300/CBP, also stimulated BSP transcription through NF-Y independently of c-jun, p300/CBP, and P/CAF. Collectively, these studies show that c-Jun and E1A regulate basal transcription of bsp in osteosarcoma cells by recruiting the NF-Y transcriptional complex to the ICE box in a mechanism that is independent of p300/CBP and P/CAF HAT activities.

Differential influence of the E4 adenoviral genes on viral and cellular promoters

BACKGROUND

Strong and stable transgene expression is fundamental to the success of recombinant adenovirus vectors in human gene therapy. However, control of transgene expression is a complex process, involving both viral and cellular factors. In this study, the influence of the E4 adenoviral region on the activity of various promoters was investigated in vitro and in vivo.

METHODS

Pairs of isogenic E1o and E1oE4o vectors were generated and compared. Levels of transgene expression were determined by Northern blot, ELISA and FACS analysis. Initiation of transcription was studied by nuclear run-on assays.

RESULTS

Similar to the viral CMV and RSV promoters, the activity of the ubiquitous cellular PGK promoter required the presence of the E4 genes in vitro and in vivo. In contrast, transgene expression from selected liver- and tumor-specific promoters did not require E4 functions.

CONCLUSION

Together with the reported low liver toxicity of E1oE4o vectors, the independence of E4 of liver-specific promoters renders such vectors interesting alternatives to the use of gutless vectors.

Oncogenic transformation-dependent expression of a transcription factor NF-Y subunit

As a result of differential splicing, one subunit of the nuclear factor Y (NF-Y) consists of two major isoforms designated short (NF-YaS) and long (NF-YaL). In proliferating normal human fibroblasts, NF-YaL is by far the more expressed isoform. Surprisingly, NF-YaS was found by immunoblotting to be as prominent as NF-YaL in simian virus 40 (SV40)-transformed cell derivatives. As a consequence, two NF-Y/DNA complexes, one containing the long and the other the short isoform, were formed with extracts from transformed cells and a target promoter element in electrophoretic mobility-shift assays. Only the complex containing NF-YaL was detected with extracts from normal fibroblasts. Furthermore, the NF-Y recognition motif contributed to promoter activation in SV40-transformed cells but not in normal, cells. Our finding links transcription stimulation in transformed cells to quantitative changes in the expression of an NF-Ya subunit.

The 9E3/CEF4 gene and its product the chicken chemotactic and angiogenic factor (cCAF): potential roles in wound healing and tumor development

The 9E3/CEF4 gene was one of the first inducible chemokine genes to be discovered. Its product, the chicken chemotactic and angiogenic factor (cCAF), is highly homologous to IL-8. It is expressed at low levels in tissues of mesenchymal origin but is highly expressed shortly after wounding and persists throughout the period of granulation tissue formation. It also is highly expressed around Rous sarcoma virus-induced tumors. The most potent natural stimulator of this gene is thrombin and in vivo cCAF is chemotactic for monocytes and lymphocytes and is angiogenic. The chemotactic properties can be potentially important in the inflammatory response and in the deterrence of tumors, whereas the angiogenic properties could be important in wound repair and tumor growth. The very rapid stimulation of 9E3 by thrombin and fast synthesis and secretion of cCAF suggest that this is a new type of stress response protein whose rapid production is designed to protect tissues rather than individual cells.

Ras-independent transformation by v-Src

Signaling by a variety of receptor and nonreceptor tyrosine kinases is mediated by Ras, a membrane-associated GTPase. Expression of v-Src, a transforming nonreceptor tyrosine kinase, results in Ras activation, and inhibition of Ras function in NIH 3T3 cells suppresses transformation by v-Src, indicating that in these cells Ras-dependent signaling pathways are required for v-Src to exert its biological effects. However, we show here that Ras was not activated in Rat-2 fibroblasts transformed by wild-type v-Src, or in chicken embryo fibroblasts transformed by SRX5, a v-Src mutant with a linker insertion at the major site of autophosphorylation. Expression of a dominant-negative mutant of Ras completely inhibited the ability of v-Src to activate the mitogen-activated protein kinase ERK2, which is downstream of Ras. However, dominant-negative Ras did not suppress transformation by v-Src as judged by a variety of criteria. Thus, v-Src can transform at least some cell types in the absence of Ras activation or Ras-stimulated ERK2 activity, and in these cells activation of Ras-independent signaling pathways must therefore be sufficient for transformation.

Chicken YB-2, a Y-box protein, is a potent activator of Rous sarcoma virus long terminal repeat-driven transcription in avian fibroblasts

We have previously reported on the cloning and characterization of chk-YB-2, a novel member of the Y-box family of proteins, that binds to the sequence 5'-GTACCACC-3' present on the noncoding strand of the Rous sarcoma virus (RSV) long terminal repeat (LTR) in a single-strand-specific manner. Here, we demonstrate that deletion or mutation of this motif not only eliminates chk-YB-2 binding in vitro but also down-regulates RSV LTR-driven transcription in avian cells. Selective abrogation of chk-YB-2 expression by using antisense oligonucleotides decreased RSV LTR-driven transcription in a promoter-specific manner. This inhibition was not observed when a reporter construct with a deletion in the chk-YB-2 binding site was used. Depletion of cellular chk-YB-2 by transfecting the cells with excess of its recognition sequence oligonucleotides also resulted in reduced transcription from the RSV LTR. Taken together, these results suggest that chk-YB-2 acts as an activator of LTR-promoted transcription in avian cells and that this activation is mediated primarily through the sequence 5'-GTACCACC-3'.

Stimulation of mouse osteopontin promoter by v-Src is mediated by a CCAAT box-binding factor

Osteopontin is an arginine-glycine-aspartic acid-containing cell adhesion protein, which is frequently expressed in transformed cells and is thought to play a role in tumorigenesis. v-Src is a transforming viral oncogene product encoded by Rous sarcoma virus (RSV). We report that v-Src expression in HT1080 fibrosarcoma cells significantly stimulates mouse osteopontin promoter activity. We also determined the v-Src response element in the osteopontin promoter as an inverted CCAAT box located at -53 to -49 from the transcription start site. Mutations of the CCAAT box disrupts protein-DNA interaction and diminishes both v-Src stimulation and basal promoter activity. A CCAAT box-containing fragment corresponding to -155 to -122 of RSV long terminal repeat competed with the -72 to -38 fragment of mouse osteopontin promoter for specific protein binding in the gel shift assay. A polyclonal antibody against CBF, a CCAAT box-binding factor, supershifted in gel shift assays the protein-DNA complex formed by nuclear extract of HT1080 with either the RSV CCAAT box fragment or with the osteopontin -72 to -38 fragment. Moreover, both osteopontin mRNA levels and enhancer activity of CCAAT box-containing -72 to -38 fragment were significantly elevated in v-src-transformed NIH 3T3 cells relative to parental cells. These findings suggest that the elevated osteopontin expression in transformed cells could be due, at least in part, to v-Src stimulation of the osteopontin promoter and that this effect is mediated by a CBF-like factor.

Cloning and characterization of mouse CCAAT binding factor

Isolation of cDNA clones for the mouse CCAAT binding factor (mCBF) has revealed the expression of two distinct forms of mCBF that are generated by alternative splicing of a single primary transcript from a gene that maps to chromosome 17. The mCBF1 mRNA encodes a protein of 997 amino acids, whereas the mCBF2 protein is predicted to be only 461 amino acids in length; mCBF1 and human CBF (hCBF) share>80% amino acid sequence identity. Analysis of adult mouse tissue RNAs has revealed that the mCBF1 and mCBF2 mRNAs are ubiquitously expressed, but that mCBF1 mRNA is 5- to 10-fold more abundant than mCBF2 mRNA. Similarly, mCBF mRNA was detected through-out the placenta and in all tissues of the developing embryo from day 8 to day 18 of gestation. Overexpression of the two forms of mCBF in mammalian cells has demonstrated that the mCBF1 and mCBF2 proteins localize to different cellular compartments, with mCBF1 found predominantly in the nucleus and mCBF2 restricted to the cytoplasm. Co-expression of these two forms influences their localization, however, indicating that CBF activity can be regulated by the relative amounts of the two forms expressed in a cell.

Transcriptional down regulation of the nov proto-oncogene in fibroblasts transformed by p60v-src

We have sought to identify genes whose expression is altered as a consequence of transformation by p60v-src. Using the mRNA differential display method, we have identified the nov proto-oncogene as one gene that is down regulated in chicken embryo fibroblasts (CEFs) transformed by p60v-src. nov transcripts were also found to be present at only very low levels in proliferating CEFs in comparison with quiescent CEFs. Serum stimulation of quiescent CEFs also resulted in a decline in the steady-state level of nov transcripts. Taken together, these findings suggest that the nov gene is expressed only in quiescent fibroblasts and that its down regulation may contribute to cellular transformation by the v-src oncogene. Down regulation of the nov gene appears to occur at both the transcriptional and posttranscriptional levels. Results obtained from experiments with a protein kinase inhibitor suggest that protein kinase C may be a key downstream effector in mediating the down regulation of nov transcripts in response to activation of p60src or serum stimulation. In addition, we found that transcription of an unknown gene is required for the decline in the steady-state level of nov transcripts in response to serum stimulation.

Introduction to the E2F family: protein structure and gene regulation

E2F is a heterodimer composed of two partners, such as E2F1 and DP1. Although E2F1 can bind DNA as a homodimer and increase promoter activity, optimal DNA-binding and transcriptional activity occurs in the heterodimeric form. A model (Fig. 3) for the involvement of E2F activity in cell growth control that incorporates viral oncoproteins, positive regulators of cell growth (cyclins) and negative regulators of cell growth (tumor suppressor proteins) can now be advanced. Each aspect of this model is addressed in subsequent chapters of this book. It is likely that binding of growth-suppressing proteins, such as Rb, can inhibit the transactivation potential of E2F1, either by blocking the interaction of E2F1 with a separate component of the transcription complex or by bringing a repressor domain to the transcription complex (Flemington et al. 1993; Helin et al. 1993; Weintraub et al. 1992; Zamanian and La Thangue 1993; Zhu et al. 1993). Phosphorylation or sequestration of Rb by viral oncoproteins can free E2F. The influence of viral oncoproteins on E2F activity and the regulation of the different E2F complexes is the focus of the contributions by Cobrinik and by Cress and Nevens. The interaction of the free E2F induces a bend in the DNA that may also play a role in transactivation, perhaps by bringing proteins (such as an Sp1 or CCAAT family member) separated by distance on the promoter DNA into contact (Huber et al. 1994). Because E2F target genes encode proteins critical for cell growth, deregulation of E2F activity can have severe consequences, such as apoptosis or uncontrolled proliferation. The effect of altered expression of E2F activity on the cell cycle and on tumorigenicity is the focus of the contribution by Adams and Kaelin. Finally, a comparison of E2F to the genetically well-characterized factors that regulate G1/S phase transcription in yeast is the subject of the chapter by Breeden. This volume concludes with Farnham's summary of the rapid gains in knowledge concerning the E2F gene family that have been made in the past several years and provides a series of questions and lines of investigation that will be the focus of future studies.

Molecular cloning and characterization of the promoter for the Chinese hamster DNA topoisomerase II alpha gene

To investigate the mechanisms governing the expression of DNA topoisomerase II alpha, the Chinese hamster topoisomerase II alpha gene has been cloned and the promoter region analyzed. There are several transcriptional start sites clustered in a region of 30 base pairs, with the major one being 102 nucleotides upstream from the ATG translation initiation site. Sequencing data reveal one GC box and a total of five inverted CCAAT elements (ICEs) within a region of 530 base pairs upstream from the major transcription start site. Sequence comparison between the human and Chinese hamster topoisomerase II alpha gene promoter regions shows a high degree of homology centered at the ICEs and GC box. In vitro DNase I footprinting results indicate protection by binding proteins at and around each ICE on both DNA strands. However, no obvious protection was observed for the GC box. Competition gel mobility shift assays with oligonucleotides containing either the wild-type or mutated ICE sequences suggest that identical or similar proteins specifically bind at each ICE, although with different affinities for individual ICE sequences. Chloramphenicol acetyltransferase assays employing nested 5'-deletions of the 5'-flanking sequence of the gene demonstrate that the sequence between -186 and +102, which contains three proximal ICEs, is sufficient for near wild-type level of promoter activity. When these three ICEs were gradually replaced with sequences which do not interact with the binding proteins, reducing promoter activity of the resulted constructs was observed. In conjunction with results from footprinting and gel mobility shift studies, the transient gene expression finding suggests that the ICEs are functionally important for the transcriptional regulation of the topoisomerase II alpha gene.

Transcriptional stimulation of the retina-specific QR1 gene upon growth arrest involves a Maf-related protein

The avian neural retina (NR) is derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of postmitotic NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-Src protein. QR1 is a retina-specific gene expressed exclusively at the stage of growth arrest and differentiation during retinal development. In NR cells infected with tsPA101, an RSV mutant conditionally defective in pp60v-src mitogenic capacity, QR1 expression is downregulated in proliferating cells at 37 degrees C and is fully restored when the cells become quiescent as a result of pp60v-src inactivation at 41 degrees C. We were able to arrest proliferation of tsPA101-infected quail NR cells expressing an active v-Src protein by serum starvation at 37 degrees C. This allowed us to investigate the role of cell growth in regulating QR1 transcription. We report that QR1 transcription is stimulated in growth-arrested cells at 37 degrees C compared with that in proliferating cells maintained at the same temperature. Growth arrest-dependent stimulation of QR1 transcription requires the integrity of the A box, a previously characterized cis-acting element responsible for QR1 transcriptional stimulation upon v-Src inactivation and during retinal differentiation. We also show that formation of the C1 complex on the A box is increased upon growth arrest by serum starvation in the presence of an active v-Src oncoprotein. Thus, the C1 complex represents an important link between cell cycle and developmental control of QR1 gene transcription during NR differentiation and RSV infection. By using antibodies directed against different Maf proteins of the leucine zipper family and competition with Maf consensus site-containing oligonucleotides in a gel shift assay, we show that the C1 complex is likely to contain a Maf-related protein. We also show that a purified bacterially expressed v-Maf protein is able to bind the A box and that the level of a 43-kDa Maf-related protein is increased upon growth arrest in infected retinal cells. Moreover, ectopic expression of c-mafI, c-mafII, and mafB cDNAs in quiescent tsPA101-infected quail NR cells is able to stimulate transcription of a QR1 reporter gene through the A box. Therefore, QR1 appears to be the first target gene for a Maf-related protein(s) in the NR.

Transcriptional downregulation of the retina-specific QR1 gene by pp60v-src and identification of a novel v-src-responsive unit

The embryonic avian neuroretina (NR) is part of the central nervous system and is composed of various cell types: photoreceptors and neuronal and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of differentiating NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-src gene. To understand the mechanisms involved in the regulation of neural cell growth and differentiation, we studied the transcriptional regulation of QR1, a gene specifically expressed in postmitotic NR cells. Transcription of this gene is detected primarily in Müller cells and is strongly downregulated by the v-src gene product. Moreover, QR1 expression takes place only during the late phase of retinal development and is shut off abruptly at hatching. We have isolated a promoter region(s) of the QR1 gene that confers v-src responsiveness. By transfection of QR1-CAT constructs into quail NR cells infected with the temperature-sensitive mutant of RSV, PA101, we have identified a v-src-responsive region located between -1208 and -1161 upstream of the transcription initiation site. This sequence is able to form two DNA-protein complexes, C1 and C2. Formation of complex C2 is specifically induced in cells expressing an active v-src product, while formation of C1 is detected mainly in nonproliferating quail NR cells upon pp60v-src inactivation. C1 is also a target for regulation during development. We have identified the DNA binding site for the C1 complex, a repeated GCTGAC sequence, and shown that mutations in this element abolish binding of this factor as well as transcription of the gene at the nonpermissive temperature. Neither formation of C1 nor that of C2 seems to involve factors known to be targeted in the pp60v-src cascade. Our data suggest that C1 could be a novel target for both developmental control and oncogene-induced cell growth regulation.

Transcriptional downregulation of the retina-specific QR1 gene by pp60v-src and identification of a novel v-src-responsive unit

The embryonic avian neuroretina (NR) is part of the central nervous system and is composed of various cell types: photoreceptors and neuronal and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of differentiating NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-src gene. To understand the mechanisms involved in the regulation of neural cell growth and differentiation, we studied the transcriptional regulation of QR1, a gene specifically expressed in postmitotic NR cells. Transcription of this gene is detected primarily in Müller cells and is strongly downregulated by the v-src gene product. Moreover, QR1 expression takes place only during the late phase of retinal development and is shut off abruptly at hatching. We have isolated a promoter region(s) of the QR1 gene that confers v-src responsiveness. By transfection of QR1-CAT constructs into quail NR cells infected with the temperature-sensitive mutant of RSV, PA101, we have identified a v-src-responsive region located between -1208 and -1161 upstream of the transcription initiation site. This sequence is able to form two DNA-protein complexes, C1 and C2. Formation of complex C2 is specifically induced in cells expressing an active v-src product, while formation of C1 is detected mainly in nonproliferating quail NR cells upon pp60v-src inactivation. C1 is also a target for regulation during development. We have identified the DNA binding site for the C1 complex, a repeated GCTGAC sequence, and shown that mutations in this element abolish binding of this factor as well as transcription of the gene at the nonpermissive temperature. Neither formation of C1 nor that of C2 seems to involve factors known to be targeted in the pp60v-src cascade. Our data suggest that C1 could be a novel target for both developmental control and oncogene-induced cell growth regulation.

Regulation of the junB gene by v-src

The proteins encoded by cellular and viral src genes are believed to be involved in the transmission of mitogenic signals, the nuclear recipients of which are largely unknown. In this work, we report that four different v-src-transformed cell lines from three different species possess elevated levels of junB transcripts. Transient expression of junB promoter-chloramphenicol acetyltransferase constructs in NIH 3T3 cells was used to demonstrate that the increase in junB transcripts was specifically associated with v-src expression and could not be recapitulated with a c-src, v-H-ras, or v-raf expression vector. Deletion mutants were used to localize the v-src-responsive region in the junB promoter to a 121-nucleotide region encompassing the CCAAT and TATAA elements. This region is distinct from one in the 5' untranslated region of the junB gene which is required to maintain its high-level basal expression. Point mutagenesis of the junB TATAA box completely abolished v-src responsiveness, suggesting that proteins which bind to this element are modified by src transformation. Several v-src and c-src mutants were used to demonstrate that elevated tyrosine kinase activity of src proteins is required for the observed effects on junB expression. Finally, homology between the TATAA box regions of junB and the unrelated but src-responsive gene 9E3/CEF-4 suggests that modulation of gene activity through proteins which bind to this region may be a recurrent, although not exclusive, theme in src transforming action. Our results suggest that src proteins may modulate some nuclear effectors through pathways not involving cellular ras or raf gene products.

Regulation of the junB gene by v-src

The proteins encoded by cellular and viral src genes are believed to be involved in the transmission of mitogenic signals, the nuclear recipients of which are largely unknown. In this work, we report that four different v-src-transformed cell lines from three different species possess elevated levels of junB transcripts. Transient expression of junB promoter-chloramphenicol acetyltransferase constructs in NIH 3T3 cells was used to demonstrate that the increase in junB transcripts was specifically associated with v-src expression and could not be recapitulated with a c-src, v-H-ras, or v-raf expression vector. Deletion mutants were used to localize the v-src-responsive region in the junB promoter to a 121-nucleotide region encompassing the CCAAT and TATAA elements. This region is distinct from one in the 5' untranslated region of the junB gene which is required to maintain its high-level basal expression. Point mutagenesis of the junB TATAA box completely abolished v-src responsiveness, suggesting that proteins which bind to this element are modified by src transformation. Several v-src and c-src mutants were used to demonstrate that elevated tyrosine kinase activity of src proteins is required for the observed effects on junB expression. Finally, homology between the TATAA box regions of junB and the unrelated but src-responsive gene 9E3/CEF-4 suggests that modulation of gene activity through proteins which bind to this region may be a recurrent, although not exclusive, theme in src transforming action. Our results suggest that src proteins may modulate some nuclear effectors through pathways not involving cellular ras or raf gene products.

Negative autoregulation of the neu gene is mediated by a novel enhancer

In an attempt to study potential feedback regulation of the neu oncogene, we have found that the neu oncogene product specifically represses its own promoter activity. Deletion analysis indicated a 140-bp region (nucleotides -312 to -173 relative to the ATG initiation codon) in the rat neu promoter responsible for neu autorepression. Gel shift assays and methylation interference analysis further demonstrated that a GGTGGGGGGG sequence (nucleotides -243 to -234 relative to the ATG initiation codon) in this 140-bp region interacts with specific protein complexes. The GGTGGGGGGG sequence (GTG element), which functions as an enhancer, is sufficient to cause neu-mediated repression in a heterologous promoter. Furthermore, it produces different gel shift patterns with nuclear extracts from neu-transformed cell lines and their parental lines, suggesting that a transcriptional factor(s) interacting with this enhancer element has been perturbed by the introduction of neu. Taken together, the data presented in this report show that (i) the neu oncogene product autorepresses its own promoter, (ii) the neu promoter contains a novel enhancer, and (iii) neu autorepression is mediated through this enhancer, likely by inhibition of the enhancer activity.

Negative autoregulation of the neu gene is mediated by a novel enhancer

In an attempt to study potential feedback regulation of the neu oncogene, we have found that the neu oncogene product specifically represses its own promoter activity. Deletion analysis indicated a 140-bp region (nucleotides -312 to -173 relative to the ATG initiation codon) in the rat neu promoter responsible for neu autorepression. Gel shift assays and methylation interference analysis further demonstrated that a GGTGGGGGGG sequence (nucleotides -243 to -234 relative to the ATG initiation codon) in this 140-bp region interacts with specific protein complexes. The GGTGGGGGGG sequence (GTG element), which functions as an enhancer, is sufficient to cause neu-mediated repression in a heterologous promoter. Furthermore, it produces different gel shift patterns with nuclear extracts from neu-transformed cell lines and their parental lines, suggesting that a transcriptional factor(s) interacting with this enhancer element has been perturbed by the introduction of neu. Taken together, the data presented in this report show that (i) the neu oncogene product autorepresses its own promoter, (ii) the neu promoter contains a novel enhancer, and (iii) neu autorepression is mediated through this enhancer, likely by inhibition of the enhancer activity.

Transcriptional activation of the CEF-4/9E3 cytokine gene by pp60v-src

The CEF-4/9E3 gene is expressed constitutively in Rous sarcoma virus (RSV)-transformed cells. This expression is largely determined by an increase in transcription of the gene. In this report, we characterize the regulatory elements responsible for the transformation-dependent activation of CEF-4/9E3. Three sequences corresponding to AP-1, PRD II/kappa B, and TAACGCAATT are involved in the process and therefore define the src-responsive unit (SRU) of the CEF-4 promoter. In constructs containing a deletion of the SRU, multiple copies of AP-1 or PRD II/kappa B, but not TAACGCAATT, led to activation of the promoter. Thus, factors interacting with these elements are constitutively activated in RSV-transformed chicken embryo fibroblasts. In agreement with the results of transient expression assays, protein binding to AP-1, PRD II/kappa B, and TAACGCAATT were more abundant in the nuclei of transformed cells. The expression of the CEF-4 promoter was investigated in cells infected by a temperature-sensitive mutant of RSV. No significant increase in CEF-4 promoter activity was detected early after activation of pp60v-src. In contrast, a substantial activation of the CEF-4 promoter was detected late after a temperature shift. Factors interacting with the TAACGCAATT, PRD II/kappa B, and AP-1 elements accumulated gradually over a period of several hours. Therefore, transcriptional activation plays an important role in the late, constitutive expression of the CEF-4 gene in stably transformed cells.

Transcriptional activation of the CEF-4/9E3 cytokine gene by pp60v-src

The CEF-4/9E3 gene is expressed constitutively in Rous sarcoma virus (RSV)-transformed cells. This expression is largely determined by an increase in transcription of the gene. In this report, we characterize the regulatory elements responsible for the transformation-dependent activation of CEF-4/9E3. Three sequences corresponding to AP-1, PRD II/kappa B, and TAACGCAATT are involved in the process and therefore define the src-responsive unit (SRU) of the CEF-4 promoter. In constructs containing a deletion of the SRU, multiple copies of AP-1 or PRD II/kappa B, but not TAACGCAATT, led to activation of the promoter. Thus, factors interacting with these elements are constitutively activated in RSV-transformed chicken embryo fibroblasts. In agreement with the results of transient expression assays, protein binding to AP-1, PRD II/kappa B, and TAACGCAATT were more abundant in the nuclei of transformed cells. The expression of the CEF-4 promoter was investigated in cells infected by a temperature-sensitive mutant of RSV. No significant increase in CEF-4 promoter activity was detected early after activation of pp60v-src. In contrast, a substantial activation of the CEF-4 promoter was detected late after a temperature shift. Factors interacting with the TAACGCAATT, PRD II/kappa B, and AP-1 elements accumulated gradually over a period of several hours. Therefore, transcriptional activation plays an important role in the late, constitutive expression of the CEF-4 gene in stably transformed cells.

CArG, CCAAT, and CCAAT-like protein binding sites in avian retrovirus long terminal repeat enhancers

A strong enhancer element is located within the long terminal repeats (LTRs) of exogenous, oncogenic avian retroviruses, such as Rous sarcoma virus (RSV) and the avian leukosis viruses. The LTRs of a second class of avian retroviruses, the endogenous viruses (evs), lack detectable enhancer function, a property that correlates with major sequence differences between the LTRs of these two virus groups. Despite this lack of independent enhancer activity, we previously identified sequences in ev LTRs that were able to functionally replace essential enhancer domains from the RSV enhancer with which they share limited sequence similarity. To identify candidate enhancer domains in ev LTRs that are functionally equivalent to those in RSV LTRs, we analyzed and compared ev and RSV LTR-specific DNA-protein interactions. Using this approach, we identified two candidate enhancer domains and one deficiency in ev LTRs. One of the proposed ev enhancer domains was identified as a CArG box, a motif also found upstream of several muscle-specific genes, and as the core sequence of the c-fos serum response element. The RSV LTR contains two CArG motifs, one at a previously identified site and one identified in this report at the same relative location as the ev CArG motif. A second factor binding site that interacts with a heat-stable protein was also identified in ev LTRs and, contrary to previous suggestions, appears to be different from previously described exogenous virus enhancer binding proteins. Finally, a deficiency in factor binding was found within the one inverted CCAAT box in ev LTRs, affirming the importance of sequences that flank CCAAT motifs in factor binding and providing a candidate defect in the ev enhancer.

Initiation of transcription from the minute virus of mice P4 promoter is stimulated in rat cells expressing a c-Ha-ras oncogene

Transformation of FR3T3 rat fibroblasts by a c-Ha-ras oncogene but not by bovine papillomavirus type 1 is associated with an increase in the abundance of mRNAs from prototype strain MVMp of infecting minute virus of mice, an oncosuppressive parvovirus. This differential parvovirus gene expression correlates with the reported sensitization of ras- but not bovine papillomavirus type 1-transformed cells to the killing effect of MVMp (N. Salomé, B. van Hille, N. Duponchel, G. Meneguzzi, F. Cuzin, J. Rommelaere, and J. Cornelis, Oncogene 5:123-130, 1990). Experiments were performed to determine at which level parvovirus expression is up-regulated in ras transformants. An MVMp "attenuation" sequence responsible for the premature arrest of RNA elongation was either placed or not placed in front of the chloramphenicol acetyltransferase gene and brought under the control of MVMp early promoter P4. Although the MVMp attenuator reduced P4-driven chloramphenicol acetyltransferase expression, the extent of attenuation was similar in normal and ras-transformed cells. Moreover, the analysis of P4-directed viral RNAs in MVMp-infected cultures by RNase protection and nuclear run-on assays also revealed a transcription elongation block of a similar amplitude in both types of cells. In addition, the stabilities of the three major parvoviral mRNAs did not vary significantly between normal and ras-transformed cells. Hence, it is concluded that the ras-induced increase in the accumulation of parvoviral mRNAs is mainly controlled at the level of transcription. Consistently, the TATA motif of the P4 promoter proved to have a differential photoreactivity when tested by in vivo UV footprinting assays in ras-transformed versus normal cells.

Transcription of muscle-specific genes is repressed by reactivation of pp60v-src in postmitotic quail myotubes

Quail myogenic cells infected with temperature sensitive (ts) mutants of Rous sarcoma virus (RSV) exhibit a temperature-dependent transformation and block of differentiation. When the cells are allowed to differentiate at the restrictive temperature (41 degrees C) and then shifted back to the permissive temperature (35 degrees C), a sharp reduction in the accumulation of muscle-specific mRNAs is observed, following reactivation of the transforming protein pp60v-src. A kinetic analysis of this down-regulation reveals that the reduction in the accumulation of muscle-specific transcripts occurs fairly rapidly within 6 to 20 h after the shift back, depending on the mRNA analyzed. Studies on transcription of endogenous muscle-specific genes and a transfected chloramphenicol acetyltransferase reporter gene under the control of muscle-specific promoters, at the different temperatures, suggest that the oncogene exerts its control mainly at the transcriptional level. On the contrary, transcription of the CMD1 gene, the avian homolog of the mouse muscle regulatory MyoD gene, is not significantly affected by the oncogene both in proliferating myoblasts and in myotubes shifted back to 35 degrees C. These findings are consistent with the conclusion that v-src blocks myogenesis by controlling transcription of muscle-specific genes independently of cell proliferation. Furthermore, they suggest the existence of an alternative pathway, not requiring the silencing of CMD1 transcription, through which the oncogene exerts its effect.

Transcription of muscle-specific genes is repressed by reactivation of pp60v-src in postmitotic quail myotubes

Quail myogenic cells infected with temperature sensitive (ts) mutants of Rous sarcoma virus (RSV) exhibit a temperature-dependent transformation and block of differentiation. When the cells are allowed to differentiate at the restrictive temperature (41 degrees C) and then shifted back to the permissive temperature (35 degrees C), a sharp reduction in the accumulation of muscle-specific mRNAs is observed, following reactivation of the transforming protein pp60v-src. A kinetic analysis of this down-regulation reveals that the reduction in the accumulation of muscle-specific transcripts occurs fairly rapidly within 6 to 20 h after the shift back, depending on the mRNA analyzed. Studies on transcription of endogenous muscle-specific genes and a transfected chloramphenicol acetyltransferase reporter gene under the control of muscle-specific promoters, at the different temperatures, suggest that the oncogene exerts its control mainly at the transcriptional level. On the contrary, transcription of the CMD1 gene, the avian homolog of the mouse muscle regulatory MyoD gene, is not significantly affected by the oncogene both in proliferating myoblasts and in myotubes shifted back to 35 degrees C. These findings are consistent with the conclusion that v-src blocks myogenesis by controlling transcription of muscle-specific genes independently of cell proliferation. Furthermore, they suggest the existence of an alternative pathway, not requiring the silencing of CMD1 transcription, through which the oncogene exerts its effect.

Phosphorylation of eukaryotic translation initiation factor 4E is increased in Src-transformed cell lines

Eukaryotic initiation factor 4F (eIF-4F) is a three-subunit complex that binds the 5' cap structure (m7GpppX, where X is any nucleotide) of eukaryotic mRNAs. This factor facilitates ribosome binding by unwinding the secondary structure in the mRNA 5' noncoding region. The limiting component of the 4F complex is believed to be the 24-kDa cap-binding phosphoprotein, eIF-4E. In this report, we describe the phosphorylation of eIF-4E in response to expression of the tyrosine kinase oncoproteins pp60v-src and pp60c-src527F. The results suggest that eIF-4E functions as a downstream target of the phosphorylation cascade induced by tyrosine-specific protein kinases as well as by effectors of the mitogenic response.

Phosphorylation of eukaryotic translation initiation factor 4E is increased in Src-transformed cell lines

Eukaryotic initiation factor 4F (eIF-4F) is a three-subunit complex that binds the 5' cap structure (m7GpppX, where X is any nucleotide) of eukaryotic mRNAs. This factor facilitates ribosome binding by unwinding the secondary structure in the mRNA 5' noncoding region. The limiting component of the 4F complex is believed to be the 24-kDa cap-binding phosphoprotein, eIF-4E. In this report, we describe the phosphorylation of eIF-4E in response to expression of the tyrosine kinase oncoproteins pp60v-src and pp60c-src527F. The results suggest that eIF-4E functions as a downstream target of the phosphorylation cascade induced by tyrosine-specific protein kinases as well as by effectors of the mitogenic response.

The Schizosaccharomyces pombe homolog of Saccharomyces cerevisiae HAP2 reveals selective and stringent conservation of the small essential core protein domain

The fission yeast Schizosaccharomyces pombe is immensely diverged from budding yeast (Saccharomyces cerevisiae) on an evolutionary time scale. We have used a fission yeast library to clone a homolog of S. cerevisiae HAP2, which along with HAP3 and HAP4 forms a transcriptional activation complex that binds to the CCAAT box. The S. pombe homolog php2 (S. pombe HAP2) was obtained by functional complementation in an S. cerevisiae hap2 mutant and retains the ability to associate with HAP3 and HAP4. We have previously demonstrated that the HAP2 subunit of the CCAAT-binding transcriptional activation complex from S. cerevisiae contains a 65-amino-acid "essential core" structure that is divisible into subunit association and DNA recognition domains. Here we show that Php2 contains a 60-amino-acid block that is 82% identical to this core. The remainder of the 334-amino-acid protein is completely without homology to HAP2. The function of php2 in S. pombe was investigated by disrupting the gene. Strikingly, like HAP2 in S. cerevisiae, the S. pombe gene is specifically involved in mitochondrial function. This contrasts to the situation in mammals, in which the homologous CCAAT-binding complex is a global transcriptional activator.

The v-src inducible gene 9E3/pCEF4 is regulated by both its promoter upstream sequence and its 3' untranslated region

The 9E3/pCEF4 mRNA is strongly induced in Rous sarcoma virus-transformed chicken embryo fibroblasts when compared to untransformed cells. To identify cis-acting transcriptional elements that confer inducibility by v-src, we isolated the 9E3 promoter upstream region. We found that 1.53 kilobases upstream of the transcriptional start site, when placed in front of a reporter gene, conferred a small degree of inducibility by v-src, in both transient and stable transfections. Two potential AP-1 sites were identified in the 9E3 promoter. AP-1 elements have been implicated previously in mediating a transcriptional response to v-src in fibroblast cell lines. These elements alone do not confer a significant inducibility by v-src in primary chicken embryo fibroblasts. Since the 9E3 mRNA is stabilized in transformed cells, we replaced the 3' untranslated region of the reporter gene with the 9E3 3' untranslated region and found this construct to be strongly responsive to stimulation by v-src. In addition, the 9E3 3' untranslated region increased the response to serum and the tumor promoter phorbol 12-myristate 13-acetate. This suggests that a posttranscriptional mechanism plays a major role in the induction of 9E3 expression.

The Schizosaccharomyces pombe homolog of Saccharomyces cerevisiae HAP2 reveals selective and stringent conservation of the small essential core protein domain

The fission yeast Schizosaccharomyces pombe is immensely diverged from budding yeast (Saccharomyces cerevisiae) on an evolutionary time scale. We have used a fission yeast library to clone a homolog of S. cerevisiae HAP2, which along with HAP3 and HAP4 forms a transcriptional activation complex that binds to the CCAAT box. The S. pombe homolog php2 (S. pombe HAP2) was obtained by functional complementation in an S. cerevisiae hap2 mutant and retains the ability to associate with HAP3 and HAP4. We have previously demonstrated that the HAP2 subunit of the CCAAT-binding transcriptional activation complex from S. cerevisiae contains a 65-amino-acid "essential core" structure that is divisible into subunit association and DNA recognition domains. Here we show that Php2 contains a 60-amino-acid block that is 82% identical to this core. The remainder of the 334-amino-acid protein is completely without homology to HAP2. The function of php2 in S. pombe was investigated by disrupting the gene. Strikingly, like HAP2 in S. cerevisiae, the S. pombe gene is specifically involved in mitochondrial function. This contrasts to the situation in mammals, in which the homologous CCAAT-binding complex is a global transcriptional activator.

Identification of cis-acting regulatory elements in the promoter region of the rat brain creatine kinase gene

The functional organization of the rat brain creatine kinase (ckb) promoter was analyzed by deletion, linker scanning, and substitution mutagenesis. Mutations were introduced into the ckb promoter of hybrid ckb/neo (neomycin resistance gene) genes, and the mutant genes were expressed transiently in HeLa cells. Expression was assayed by primer extension analysis of neo RNA, which allowed the transcription start sites and the amount of transcription to be determined. Transfections and primer extension reactions were internally controlled by simultaneous analysis of transcription from the adenovirus VA gene located on the same plasmid as the hybrid ckb/neo gene. We demonstrate that 195 bp of the ckb promoter is sufficient for efficient in vivo expression in HeLa cells. A nonconsensus TTAA element at -28 bp appears to provide the TATA box function for the ckb promoter in vivo. Two CCAAT elements, one at -84 bp and the other at -54 bp, and a TATAAA TA element (a consensus TATA box sequence) at -66 bp are required for efficient transcription from the TTAA element. In addition, we present evidence that the consensus beta-globin TATA box responds to the TATAAATA element in the same way as the ckb nonconsensus TTAA element.

A cloned human CCAAT-box-binding factor stimulates transcription from the human hsp70 promoter

The basal promoter of the human hsp70 gene is predominantly controlled by a CCAAT element at position -70 relative to the transcriptional initiation site. We report the isolation of a novel cDNA clone encoding a 114-kDa polypeptide that binds to the CCAAT element of the hsp70 promoter. Expression of this CCAAT-binding factor (CBF) cDNA activated transcription from cotransfected hsp70 promoter-reporter gene constructs in a CCAAT-dependent manner. CCAAT-binding factor shows no homology to the previously identified human CCAAT transcription factor or rat CCAAT/enhancer-binding protein.

A cloned human CCAAT-box-binding factor stimulates transcription from the human hsp70 promoter

The basal promoter of the human hsp70 gene is predominantly controlled by a CCAAT element at position -70 relative to the transcriptional initiation site. We report the isolation of a novel cDNA clone encoding a 114-kDa polypeptide that binds to the CCAAT element of the hsp70 promoter. Expression of this CCAAT-binding factor (CBF) cDNA activated transcription from cotransfected hsp70 promoter-reporter gene constructs in a CCAAT-dependent manner. CCAAT-binding factor shows no homology to the previously identified human CCAAT transcription factor or rat CCAAT/enhancer-binding protein.

Identification of cis-acting regulatory elements in the promoter region of the rat brain creatine kinase gene

The functional organization of the rat brain creatine kinase (ckb) promoter was analyzed by deletion, linker scanning, and substitution mutagenesis. Mutations were introduced into the ckb promoter of hybrid ckb/neo (neomycin resistance gene) genes, and the mutant genes were expressed transiently in HeLa cells. Expression was assayed by primer extension analysis of neo RNA, which allowed the transcription start sites and the amount of transcription to be determined. Transfections and primer extension reactions were internally controlled by simultaneous analysis of transcription from the adenovirus VA gene located on the same plasmid as the hybrid ckb/neo gene. We demonstrate that 195 bp of the ckb promoter is sufficient for efficient in vivo expression in HeLa cells. A nonconsensus TTAA element at -28 bp appears to provide the TATA box function for the ckb promoter in vivo. Two CCAAT elements, one at -84 bp and the other at -54 bp, and a TATAAA TA element (a consensus TATA box sequence) at -66 bp are required for efficient transcription from the TTAA element. In addition, we present evidence that the consensus beta-globin TATA box responds to the TATAAATA element in the same way as the ckb nonconsensus TTAA element.