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

JunD/AP-1 Antagonizes the Induction of DAPK1 To Promote the Survival of v-Src-Transformed Cells

The increase in AP-1 activity is a hallmark of cell transformation by tyrosine kinases. Previously, we reported that blocking AP-1 using the c-Jun dominant negative mutant TAM67 induced senescence, adipogenesis, or apoptosis in v-Src-transformed chicken embryo fibroblasts (CEFs) whereas inhibition of JunD by short hairpin RNA (shRNA) specifically induced apoptosis. To investigate the role of AP-1 in Src-mediated transformation, we undertook a gene profiling study to characterize the transcriptomes of v-Src-transformed CEFs expressing either TAM67 or the JunD shRNA. Our study revealed a cluster of 18 probe sets upregulated exclusively in response to AP-1/JunD impairment and v-Src transformation. Four of these probe sets correspond to genes involved in the interferon pathway. One gene in particular, death-associated protein kinase 1 (DAPK1), is a C/EBPβ-regulated mediator of apoptosis in gamma interferon (IFN-γ)-induced cell death. Here, we show that inhibition of DAPK1 abrogates cell death in v-Src-transformed cells expressing the JunD shRNA. Chromatin immunoprecipitation data indicated that C/EBPβ was recruited to the DAPK1 promoter while the expression of a dominant negative mutant of C/EBPβ abrogated the induction of DAPK1 in response to the inhibition of AP-1. In contrast, as determined by chromatin immunoprecipitation (ChIP) assays, JunD was not detected on the DAPK1 promoter under any conditions, suggesting that JunD promotes survival by indirectly antagonizing the expression of DAPK1 in v-Src transformed cells.

IMPORTANCE

Transformation by the v-Src oncoprotein causes extensive changes in gene expression in primary cells such as chicken embryo fibroblasts. These changes, determining the properties of transformed cells, are controlled in part at the transcriptional level. Much attention has been devoted to transcription factors such as AP-1 and NF-κB and the control of genes associated with a more aggressive phenotype. In this report, we describe a novel mechanism of action determined by the JunD component of AP-1, a factor enhancing cell survival in v-Src-transformed cells. We show that the loss of JunD results in the aberrant activation of a genetic program leading to cell death. This program requires the activation of the tumor suppressor death-associated protein kinase 1 (DAPK1). Since DAPK1 is phosphorylated and inhibited by v-Src, these results highlight the importance of this kinase and the multiple mechanisms controlled by v-Src to antagonize the tumor suppressor function of DAPK1.

Extracellular Signal-Regulated Kinase 2 and CHOP Restrict the Expression of the Growth Arrest-Specific p20K Lipocalin Gene to G0

The activation of the growth arrest-specific (gas) p20K gene depends on the interaction of C/EBPβ with two elements of a 48-bp promoter region termed the quiescence-responsive unit (QRU). Here we identify extracellular signal-related kinase 2 (ERK2) as a transcriptional repressor of the p20K QRU in cycling chicken embryo fibroblasts (CEF). ERK2 binds to repeated GAAAG sequences overlapping the C/EBPβ sites of the QRU. The recruitment of ERK2 and C/EBPβ is mutually exclusive and dictates the expression of p20K. C/EBP homologous protein (CHOP) was associated with C/EBPβ under conditions promoting endoplasmic reticulum (ER) stress and, to a lesser extent, in cycling CEF but was not detectable when C/EBPβ was immunoprecipitated from contact-inhibited cells. During ER stress, overexpression of CHOP inhibited p20K, while its downregulation promoted p20K, indicating that CHOP is also a potent inhibitor of p20K. Transcriptome analyses revealed that hypoxia-responsive genes are strongly induced in contact-inhibited but not serum-starved CEF, and elevated levels of nitroreductase activity, a marker of hypoxia, were detected at confluence. Conditions of hypoxia (2% O₂) induced growth arrest in subconfluent CEF and markedly stimulated p20K expression, suggesting that the control of proliferation and gas gene expression is closely linked to limiting oxygen concentrations associated with high cell densities.

In vivo imaging of transiently transgenized mice with a bovine interleukin 8 (CXCL8) promoter/luciferase reporter construct

One of the most remarkable properties of interleukin 8 (CXCL8/IL-8), a chemokine with known additional functions also in angiogenesis and tissue remodeling, is the variation of its expression levels. In healthy tissues, IL-8 is barely detectable, but it is rapidly induced by several folds in response to proinflammatory cytokines, bacterial or viral products, and cellular stress. Although mouse cells do not bear a clear homologous IL-8 gene, the murine transcriptional apparatus may well be capable of activating or repressing a heterologous IL-8 gene promoter driving a reporter gene. In order to induce a transient transgenic expression, mice were systemically injected with a bovine IL-8 promoter–luciferase construct. Subsequently mice were monitored for luciferase expression in the lung by in vivo bioluminescent image analysis over an extended period of time (up to 60 days). We demonstrate that the bovine IL-8 promoter–luciferase construct is transiently and robustly activated 3–5 hours after LPS and TNF-α instillation into the lung, peaking at 35 days after construct delivery. Bovine IL-8 promoter–luciferase activation correlates with white blood cell and neutrophil infiltration into the lung. This study demonstrates that a small experimental rodent model can be utilized for non-invasively monitoring, through a reporter gene system, the activation of an IL-8 promoter region derived from a larger size animal (bovine). This proof of principle study has the potential to be utilized also for studying primate IL-8 promoter regions.

Pleiotropic action of AP-1 in v-Src-transformed cells

The activation of AP-1 is a hallmark of cell transformation by tyrosine kinases. In this study, we characterize the role of AP-1 proteins in the transformation of chicken embryo fibroblasts (CEF) by v-Src. In normal CEF, the expression of a dominant negative mutant of c-Jun (TAM67) induced senescence. In contrast, three distinct phenotypes were observed when TAM67 was expressed in v-Src-transformed CEF. While senescent cells were also present, the inhibition of AP-1 caused apoptosis in a fraction of the v-Src-transformed cells. In addition, cells containing lipid-rich vesicles accumulated, suggesting that a subpopulation of the v-Src-transformed cells underwent differentiation in response to the inhibition of AP-1. JunD and Fra-2 were the main components of this factor, while c-Jun accounted for a minor fraction of AP-1 in v-Src-transformed CEF. The downregulation of c-Jun expression by short hairpin RNA (shRNA) induced senescence in normal and v-Src-transformed cells. In contrast, a high incidence of apoptosis was caused by the downregulation of JunD, suggesting that it is required for the survival of v-Src-transformed CEF. Levels of the p53 tumor suppressor were elevated under conditions of JunD inhibition. Repression of p53 by shRNA enhanced the survival and anchorage-independent proliferation of v-Src-transformed CEF with JunD/AP-1 inhibition. The inhibition of Fra-2 had no visible phenotype in normal CEF but caused the appearance of lipid-rich vesicles in v-Src-transformed CEF. Therefore, AP-1 facilitated transformation by acting as a survival factor, by inhibiting premature entry into senescence, and by blocking the differentiation of v-Src-transformed CEF.

The constitutive activation of the CEF-4/9E3 chemokine gene depends on C/EBPbeta in v-src transformed chicken embryo fibroblasts

The CEF-4/9E3 chemokine gene is expressed constitutively in chicken embryo fibroblasts (CEF) transformed by the Rous sarcoma virus (RSV). This aberrant induction is controlled at the transcriptional and post-transcriptional levels. Transcriptional activation depends on multiple elements of the CEF-4 promoter composing a Src-responsive-Unit or SRU. The SRU includes a TPA responsive element, a PRDII/kappaB domain and a CAAT box. In this report, we identify C/EBPbeta as a component of the trans-acting factor interacting with the CAAT box of the CEF-4 promoter. In addition, we show that C/EBPbeta binds to a second element located in proximity of the TRE. A mutation of this distal CAAT box impaired the activation of the CEF-4 promoter by pp60(v-src) indicating that this element is also part of the SRU. Using the RCASBP retroviral vector, we expressed a dominant negative mutant of C/EBPbeta (designated Delta184-C/EBPbeta) in RSV-transformed CEF. Delta184-C/EBPbeta decreased the accumulation of the CEF-4 mRNA and activation of the CEF-4 promoter by pp60(v-src). The induction of the Cox-2 gene (CEF-147) was also reduced by Delta184-C/EBPbeta. The effect of the dominant negative mutant was observed within 1 h of the activation of a thermolabile pp60(v-src) suggesting that C/EBPbeta is an early target of v-src transformation. The dominant negative mutant did not inhibit the transformation of CEF by RSV and in fact accentuated the transformed cell phenotype. Therefore, the activation of C/EBPbeta is important for the expression of v-src regulated genes but is not required for the in vitro transformation of CEF by pp60(v-src).

C/EBPbeta (NF-M) is essential for activation of the p20K lipocalin gene in growth-arrested chicken embryo fibroblasts

The p20K gene is induced in conditions of reversible growth arrest in chicken embryo fibroblasts (CEF). This expression is dependent on transcriptional activation and on a region of the promoter designated the quiescence-responsive unit (QRU). In this report, we describe the regulatory elements of the QRU responsible for activation in resting cells and characterize the trans-acting proteins interacting with these elements. We show that the QRU consists of functionally distinct domains including quiescence-specific and weak proliferation-responsive elements. The quiescence responsiveness of the QRU was mapped to two C/EBP binding sites, and the activity of the p20K promoter and its QRU was inhibited by the expression of a dominant negative mutant of C/EBPbeta in nondividing cells. The activation of QRU in response to serum starvation and contact inhibition correlated with the presence of a growth arrest-specific complex in electrophoretic mobility shift assays. This complex was supershifted by antibody for C/EBPbeta. C/EBPbeta accumulated in conditions of contact inhibition as a result of transcriptional activation. Therefore, C/EBPbeta was itself regulated as a growth arrest-specific gene in CEF. Finally, we show that the expression of p20K is regulated by linoleic acid, an essential fatty acid binding to p20K. The addition of linoleic acid to contact-inhibited CEF markedly repressed the synthesis of p20K without inducing mitogenesis. The activity of the QRU was inhibited by linoleic acid or the peroxisome proliferator-activated receptor PPARgamma2 in transient expression assays. Therefore, we have identified C/EBPbeta as a key activator of a growth arrest-specific gene in CEF and implicated an essential fatty acid, linoleic acid, in regulation of the QRU and the p20K lipocalin gene.

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.

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.

v-src induction of the TIS10/PGS2 prostaglandin synthase gene is mediated by an ATF/CRE transcription response element

We recently reported the cloning of a mitogen-inducible prostaglandin synthase gene, TIS10/PGS2. In addition to growth factors and tumor promoters, the v-src oncogene induces TIS10/PGS2 expression in 3T3 cells. Deletion analysis, using luciferase reporters, identifies a region between -80 and -40 nucleotides 5' of the TIS10/PGS2 transcription start site that mediates pp60v-src induction in 3T3 cells. This region contains the sequence CGTCACGTG, which includes overlapping ATF/CRE (CGTCA) and E-box (CACGTG) sequences. Gel shift-oligonucleotide competition experiments with nuclear extracts from cells stably transfected with a temperature-sensitive v-src gene demonstrate that the CGTCACGTG sequence can bind proteins at both the ATF/CRE and E-box sequences. Dominant-negative CREB and Myc proteins that bind DNA, but do not transactivate, block v-src induction of a luciferase reporter driven by the first 80 nucleotides of the TIS10/PGS2 promoter. Mutational analysis distinguishes which TIS10/PGS2 cis-acting element mediates pp60v-src induction. E-box mutation has no effect on the fold induction in response to pp60v-src. In contrast, ATF/CRE mutation attenuates the pp60v-src response. Antibody supershift and methylation interference experiments demonstrate that CREB and at least one other ATF transcription factor in these extracts bind to the TIS10/PGS2 ATF/CRE element. Expression of a dominant-negative ras gene also blocks TIS10/PGS2 induction by v-src. Our data suggest that Ras mediates pp60v-src activation of an ATF transcription factor, leading to induced TIS10/PGS2 expression via the ATF/CRE element of the TIS10/PGS2 promoter. This is the first description of v-src activation of gene expression via an ATF/CRE element.

v-src induction of the TIS10/PGS2 prostaglandin synthase gene is mediated by an ATF/CRE transcription response element

We recently reported the cloning of a mitogen-inducible prostaglandin synthase gene, TIS10/PGS2. In addition to growth factors and tumor promoters, the v-src oncogene induces TIS10/PGS2 expression in 3T3 cells. Deletion analysis, using luciferase reporters, identifies a region between -80 and -40 nucleotides 5' of the TIS10/PGS2 transcription start site that mediates pp60v-src induction in 3T3 cells. This region contains the sequence CGTCACGTG, which includes overlapping ATF/CRE (CGTCA) and E-box (CACGTG) sequences. Gel shift-oligonucleotide competition experiments with nuclear extracts from cells stably transfected with a temperature-sensitive v-src gene demonstrate that the CGTCACGTG sequence can bind proteins at both the ATF/CRE and E-box sequences. Dominant-negative CREB and Myc proteins that bind DNA, but do not transactivate, block v-src induction of a luciferase reporter driven by the first 80 nucleotides of the TIS10/PGS2 promoter. Mutational analysis distinguishes which TIS10/PGS2 cis-acting element mediates pp60v-src induction. E-box mutation has no effect on the fold induction in response to pp60v-src. In contrast, ATF/CRE mutation attenuates the pp60v-src response. Antibody supershift and methylation interference experiments demonstrate that CREB and at least one other ATF transcription factor in these extracts bind to the TIS10/PGS2 ATF/CRE element. Expression of a dominant-negative ras gene also blocks TIS10/PGS2 induction by v-src. Our data suggest that Ras mediates pp60v-src activation of an ATF transcription factor, leading to induced TIS10/PGS2 expression via the ATF/CRE element of the TIS10/PGS2 promoter. This is the first description of v-src activation of gene expression via an ATF/CRE element.

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.

V-src-induced-transcription of the avian clusterin gene

We have isolated the avian gene T64 corresponding to the mammalian clusterin, on the basis of high accumulation of its template mRNA in cells infected with oncogenic retroviruses. Since the clusterin was shown to have a protective effect against the immune system, its induction by oncogenic viruses is of major biological importance. The unique, short 5 kb-long T64 genomic locus is inactive in normal quail embryo fibroblasts in primary culture whereas it shows a high transcriptional activity after transformation by the Rous sarcoma virus. The 963 bp-long 5' flanking region is sufficient to drive the transcription of the chloramphenicol acetyltransferase reporter gene in a thermodependent manner when a thermosensitive version of pp60v-src is used. Deletion and point mutation analyses of the promoter show that the v-src response requires at least two separate elements: PUR and AP-1, located respectively at positions -167 to -152 and -25 to -19 relative to the single transcription initiation site. In addition, the binding of specific nuclear factors to these responsive elements correlates with the T64 promoter activation.