Transformation and transactivation suppressor activity of the c-Jun leucine zipper fused to a bacterial repressor

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.

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.

Cellular processes of v-Src transformation revealed by gene profiling of primary cells--implications for human cancer

Background

Cell transformation by the Src tyrosine kinase is characterized by extensive changes in gene expression. In this study, we took advantage of several strains of the Rous sarcoma virus (RSV) to characterize the patterns of v-Src-dependent gene expression in two different primary cell types, namely chicken embryo fibroblasts (CEF) and chicken neuroretinal (CNR) cells. We identified a common set of v-Src regulated genes and assessed if their expression is associated with disease-free survival using several independent human tumor data sets.

Methods

CEF and CNR cells were infected with transforming, non-transforming, and temperature sensitive mutants of RSV to identify the patterns of gene expression in response to v-Src-transformation. Microarray analysis was used to measure changes in gene expression and to define a common set of v-Src regulated genes (CSR genes) in CEF and CNR cells. A clustering enrichment regime using the CSR genes and two independent breast tumor data-sets was used to identify a 42-gene aggressive tumor gene signature. The aggressive gene signature was tested for its prognostic value by conducting survival analyses on six additional tumor data sets.

Results

The analysis of CEF and CNR cells revealed that cell transformation by v-Src alters the expression of 6% of the protein coding genes of the genome. A common set of 175 v-Src regulated genes (CSR genes) was regulated in both CEF and CNR cells. Within the CSR gene set, a group of 42 v-Src inducible genes was associated with reduced disease- and metastasis-free survival in several independent patient cohorts with breast or lung cancer. Gene classes represented within this group include DNA replication, cell cycle, the DNA damage and stress responses, and blood vessel morphogenesis.

Conclusion

By studying the v-Src-dependent changes in gene expression in two types of primary cells, we identified a set of 42 inducible genes associated with poor prognosis in breast and lung cancer. The identification of these genes provides a set of biomarkers of aggressive tumor behavior and a framework for the study of cancer cells characterized by elevated Src kinase activity.

Design of a modular immunotoxin connected by polyionic adapter peptides

Immunotoxins are genetically engineered fusion proteins of an antibody Fv fragment and a toxin from bacteria or plants, which function as anti-cancer therapeutics. Here, we describe a new generation of immunotoxins in which both proteins do not form a single fusion protein but are coupled specifically via cysteine-containing polyionic fusion peptides. The engineered Pseudomonas exotoxin PE38 was N-terminally fused to the peptide E(8)C. In combination with the disulfide-stabilized Fv fragment of the tumor-specific antibody B3, which was extended by the peptide R(8)CP, the fusion peptides ensured a specific and covalent coupling of the Fv fragment and the toxin. The resulting immunotoxin was as active and as specific as an immunotoxin consisting of a fusion protein of the same antibody fragment connected to the toxin.

Raf-independent deregulation of p38 and JNK mitogen-activated protein kinases are critical for Ras transformation

Activated Ras, but not Raf, causes transformation of RIE-1 epithelial cells, supporting the importance of Raf-independent pathways in mediating Ras transformation. The p38 and JNK mitogen-activated protein kinase cascades are activated by Ras via Raf-independent effector function. Therefore, we determined whether p38 and JNK activation are involved in Ras transformation of RIE-1 epithelial cells. Rather surprisingly, we found that pharmacologic inhibition of p38, together with Raf activation of ERK, was sufficient to mimic the morphologic and growth transformation caused by oncogenic Ras. p38 inhibition together with ERK activation also caused the same alterations in cyclin D1 and p21(CIP1) expression caused by Ras and induced an autocrine growth factor loop important for transformation. Finally, in contrast to p38, we found that JNK activation promoted Ras transformation, and that Ras deregulation of p38 and JNK was not mediated by activation of the Rac small GTPase. We conclude that a key action of Raf-independent effector pathways important for Ras transformation may involve inhibition of p38 and activation of JNK.

Loss of transgelin in breast and colon tumors and in RIE-1 cells by Ras deregulation of gene expression through Raf-independent pathways

Activated Ras but not Raf can transform RIE-1 and other epithelial cells, indicating the critical importance of Raf-independent effector function in Ras transformation of epithelial cells. To elucidate the nature of these Raf-independent activities, we utilized representational difference analysis to identify genes aberrantly expressed by Ras through Raf-independent mechanisms in RIE-1 cells. We identified a total of 22 genes, both known and novel, whose expression was either activated or abolished by Ras but not Raf. The genes up-regulated encode proteins involved in protein or DNA synthesis, regulation of protease activity, or ligand binding, whereas those genes down-regulated encode actin cytoskeletal-, extracellular matrix-, and gap junction-associated proteins, and transmembrane receptor- or cytokine-like proteins. These results suggest that a key function of Raf-independent signaling involves deregulation of gene expression. We further characterized transgelin as a gene whose expression was abolished by Ras. Transgelin was identified previously as a protein whose expression was lost in virally transformed cell lines. We show that this loss is regulated at the level of gene expression and that both Raf-dependent and Raf-independent pathways are required to cause Ras down-regulation of transgelin in RIE-1 cells, whereas Raf alone is sufficient to cause its loss in NIH 3T3 fibroblasts. We also found that Ras-dependent and Ras-independent mechanisms can cause the down-regulation of transgelin in human breast and colon carcinoma cells lines and patient-derived tumor samples. We conclude that loss of transgelin gene expression may be an important early event in tumor progression and a diagnostic marker for breast and colon cancer development.

Polyionic fusion peptides function as specific dimerization motifs

The de novo design of a molecular adapter for directed association and covalent linkage of two polypeptides is presented. Using peptides containing charged amino acid residues and an additional cysteine residue (AlaCysLys(8) and AlaCysGlu(8)) we demonstrate that the electrostatic interaction promotes the association of two synthetic peptides and, subsequently, disulfide bond formation. The reaction depends on both the redox potential and on the ionic strength of the buffer. Varying the redox potential, the interaction of the peptides was quantified by a Delta G(0') of 6.6 +/- 0.2 kcal/mol. Heterodimerization of the peptides is highly specific, a competition of association by other cysteine containing compounds could not be observed. Two proteins comprising cysteine-containing polyionic fusion peptides, a modified Fab fragment and an alpha-glucosidase fusion, could be specifically conjugated by directed association and subsequent disulfide bond formation. Both proteins retain their functional characteristics within the bifunctional conjugate: enzymatic activity of the alpha-glucosidase and antigen-binding capacity of the Fab fragment are equivalent to the non-conjugated components.

Identification of the domains of UreR, an AraC-like transcriptional regulator of the urease gene cluster in Proteus mirabilis

Proteus mirabilis urease catalyzes the hydrolysis of urea to CO(2) and NH(3), resulting in urinary stone formation in individuals with complicated urinary tract infections. UreR, a member of the AraC family, activates transcription of the genes encoding urease enzyme subunits and accessory proteins, ureDABCEFG, as well as its own transcription in the presence of urea. Based on sequence homology with AraC, we hypothesized that UreR contains both a dimerization domain and a DNA-binding domain. A translational fusion of the leucine zipper dimerization domain (amino acids 302 to 350) of C/EBP and the C-terminal half of UreR (amino acids 164 to 293) activated transcription from the ureD promoter (p(ureD)) and bound to a 60-bp fragment containing p(ureD), as analyzed by gel shift. These results were consistent with the DNA-binding specificity residing in the C-terminal half of UreR and dimerization being required for activity. To localize the dimerization domain of UreR, a translational fusion of the DNA-binding domain of the LexA repressor (amino acids 1 to 87) and the N-terminal half of UreR (amino acids 1 to 182) was constructed and found to repress transcription from p(sulA)-lacZ (sulA is repressed by LexA) and bind to the sulA operator site, as analyzed by gel shift. Since LexA binds this site only as a dimer, the UreR(1-182)-LexA(1-87) fusion also must dimerize to bind p(sulA). Indeed, purified UreR-Myc-His eluted from a gel filtration column as a dimer. Therefore, we conclude that the dimerization domain of UreR is located within the N-terminal half of UreR. UreR contains three leucines that mimic the leucines that contribute to dimerization of AraC. Mutagenesis of Leu147, Leu148, or L158 alone did not significantly affect UreR function. In contrast, mutagenesis of both Leu147 and Leu148 or all three Leu residues resulted in a 85 or 94% decrease, respectively, in UreR function in the presence of urea (P < 0.001). On the contrary, His102 and His175 mutations of UreR resulted in constitutive induction in the absence of urea. We conclude that a dimerization domain resides in the N-terminal half of the polypeptide, that Leu residues may contribute to this function, and that sequences within the C-terminal half of UreR are responsible for DNA binding to the urease promoter regions. Selected His residues also contribute significantly to UreR function.

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).

Endogenous AP-1 levels necessary for oncogenic activity are higher than those sufficient to support normal growth

We investigated the role of endogenous AP-1 in human tumor cell lines by introducing SupJunD-1, a dominant-negative mutant of AP-1, using vesicular stomatitis virus G protein (VSV-G)-pseudotyped retrovirus vectors. Single inoculation of six human tumor cell lines, originating from osteosarcomas, non-small cell lung carcinomas or cervical carcinomas, with recombinant SupJunD-1 virus at a high multiplicity of infection readily inhibited colony formation in soft agar. We detected no significant changes in expression levels of AP-1 components c-Jun or Fra-1, adhesion molecules CD44 or E-cadherin, or cell cycle regulator p53, which are encoded by genes previously reported to be under the control of AP-1 in some mouse or human cell lines. By varying the dosage of VSV-G-pseudotyped retrovirus, we were able to change the proviral copy number of supjunD-1 from 1 to approximately 10 and monitor suppression of endogenous AP-1 function as assessed by growth characteristics of the tumor cell lines, we found a SupJunD-1 dosage which significantly suppressed anchorage-independent growth without affecting the cellular growth in monolayer cultures at all. We conclude that endogenous AP-1 levels necessary for oncogenic activity are much higher than those sufficient to support normal growth.

Directed mutation of the basic domain of v-Jun alters DNA binding specificity and abolishes its oncogenic activity in chicken embryo fibroblasts

Overexpression of v-Jun in chicken embryo fibroblasts (CEF) leads to oncogenic transformation phenotypically characterized by anchorage independent growth and release from contact inhibition (focus formation). The mechanisms involved in this oncogenic conversion however, are not yet clear. Because Jun is a transcription factor, it has been assumed that oncogenic transformation results directly from deregulated AP-1 target gene expression. However, a number of experimental observations in avian cell culture models fail to correlate oncogenesis with AP-1 activity suggesting that transformation induced by v-Jun may occur through an indirect mechanism. To test this possibility, we introduced point mutations into the basic DNA binding domain of v-Jun and created mutants that exhibit altered binding specificity. When expressed in CEF, these mutants fail to deregulate three known v-Jun target genes (JTAP-1, apolipoprotein A1, c-Jun) thus demonstrating in vivo specificity changes. Each of the binding specificity mutants was also tested for its ability to induce oncogenic transformation. Interestingly, expression of these mutants in CEF results in a phenotype indistinguishable from the vector control with respect to growth rate, focus formation and the ability to form colonies in soft agar. These results are consistent with a model requiring direct AP-1 target deregulation as a prerequisite of v-Jun induced cell transformation. With this in mind, we generated a series of additional mutants that retain the ability to bind AP-1 sequence elements, but vary in their oncogenic potential. We demonstrate the use of these mutants to screen v-Jun induced gene targets for a functional role in cell transformation.

Characterization of the fatty acid-responsive transcription factor FadR. Biochemical and genetic analyses of the native conformation and functional domains

In Escherichia coli, fatty acid synthesis and degradation are coordinately controlled at the level of transcription by FadR. FadR represses transcription of at least eight genes required for fatty acid transport and beta-oxidation and activates transcription of at least two genes required for unsaturated fatty acid biosynthesis and the gene encoding the transcriptional regulator of the aceBAK operon encoding the glyoxylate shunt enzymes, IclR. FadR-dependent DNA binding and transcriptional activation is prevented by long chain fatty acyl-CoA. In the present work, we provide physical and genetic evidence that FadR exists as a homodimer in solution and in vivo. Native polyacrylamide gel electrophoresis and glycerol gradient ultracentrifugation of the purified protein show that native FadR was a homodimer in solution with an apparent molecular mass of 53.5 and 57.8 kDa, respectively. Dominant negative mutations in fadR were generated by random and site-directed mutagenesis. Each mutation mapped to the amino terminus of the protein (residues 1-66) and resulted in a decrease in DNA binding in vitro. In an effort to separate domains of FadR required for DNA binding, dimerization, and ligand binding, chimeric protein fusions between the DNA binding domain of LexA and different regions of FadR were constructed. One fusion, LexA1-87-FadR102-239, was able to repress the LexA reporter sulA-lacZ, and beta-galactosidase activities were derepressed by fatty acids, suggesting that the fusion protein had determinants both for dimerization and ligand binding. These studies support the conclusion that native FadR exists as a stable homo-dimer in solution and that determinants for DNA binding and acyl-CoA binding are found within the amino terminus and carboxyl terminus, respectively.

Requirement of NF-kappaB activation to suppress p53-independent apoptosis induced by oncogenic Ras

The ras proto-oncogene is frequently mutated in human tumors and functions to chronically stimulate signal transduction cascades resulting in the synthesis or activation of specific transcription factors, including Ets, c-Myc, c-Jun, and nuclear factor kappa B (NF-kappaB). These Ras-responsive transcription factors are required for transformation, but the mechanisms by which these proteins facilitate oncogenesis have not been fully established. Oncogenic Ras was shown to initiate a p53-independent apoptotic response that was suppressed through the activation of NF-kappaB. These results provide an explanation for the requirement of NF-kappaB for Ras-mediated oncogenesis and provide evidence that Ras-transformed cells are susceptible to apoptosis even if they do not express the p53 tumor-suppressor gene product.

Communication between the extracellular environment, cytoplasmic signalling cascades and the nuclear cell-cycle machinery

In the past decade, we have gained considerable insight into the identities of various cytoplasmic signal transduction cascades and the manner in which they operate in response to changes in the extracellular environment. Moreover, we have begun to understand what the key players are in cell-cycle regulation and how they, in turn, function to promote cell division. A long-standing question, however, has been how communication between signalling routes and the cell-cycle machinery occurs. This review highlights some recent observations that provide possible links between signal transduction and the cell-cycle machinery.

Attenuated stress responses in young and old human lymphocytes

Aging generally is understood to be a period defined by altered responses to physiological stress. At the molecular level, several stress responses involving specific gene expression have been revealed, and thermal stress has been tightly linked to induction of the heat shock gene family (D.A. Jurivich. In E. Bittar (ed.), Principles of Medical Biology, Vol. 4, JAI press, San Diego, 1996, pp. 411-462). Perturbations in heat shock gene transcription consistently have been noted in senescent cells from all species examined thus far. Because heat shock proteins serve several vital functions in the immune system, changes in the thermal stress response could potentially contribute to immunosenescence. Inadequate promoter priming by the transactivator or heat shock genes, heat shock factor 1 (HSF1), is thought to account for age-dependent diminution in expression of these genes, although the exact mechanism for this loss is not clearly understood. We have found that human lymphocytes exhibit an age-dependent loss in HSF1-DNA binding, although a range of binding has been observed in both young and old donor cells. This report characterizes a subset of young and old human donor lymphocytes that are non-responders to thermal stress defined by the absence of HSF1-DNA binding after a 42 degrees C heat shock. Whole cell extracts from these donor cells have the capacity to inhibit HSF1-DNA binding when mixed with pre-activated HSF1 from HeLa cells. This inhibitory activity is lost upon heat denaturation and does not appear to be protease mediated. Serial passage of lymphoblasts recapitulates loss of heat inducible HSF1-DNA observed in old donor lymphocytes, thus suggesting that loss of replicative potential and aging lead to altered stress responses. Uncoupling of the thermal response and its potential relevance to apoptosis and aging are discussed.

p120 GAP modulates Ras activation of Jun kinases and transformation

Although recent evidence demonstrates that Ras causes transformation by activation of multiple downstream pathways, the specific role of non-Raf effector pathways is presently unknown. Although Ras causes activation of the Jun NH2-terminal kinases (JNKs) via a Raf-independent pathway, the contribution of JNK activation to Ras transformation and the effector that mediates JNK activation have not been established. We observed that a dominant negative mutant of SEK1/JNKK, an activator of JNKs, selectively inhibited oncogenic Ras activation of JNK and Ras transformation, but not Ras activation of the p42 mitogen-activated protein kinase. In contrast, overexpression of wild type SEK1 enhanced Ras activation of JNK and transforming activity. Thus, JNK activation promotes Ras transformation. Furthermore, a dominant negative mutant of p120 GAP (designated N-GAP), a candidate Ras effector, blocked Ras, but not Raf, transformation and blocked Ras, but not Rac, activation of JNK. Since N-GAP overexpression reduced the association of p190 Rac/Rho GAP with endogenous p120 GAP, N-GAP may form nonproductive complexes with components critical for p120 GAP function. In summary, p120 GAP may function as an effector for Ras activation of JNK and Ras transformation.

Multiple signaling pathways control the activation of the CEF-4/9E3 cytokine gene by pp60v-src

The CEF-4/9E3 cytokine gene is expressed aberrantly in chicken embryo fibroblasts (CEF) transformed by the Rous sarcoma virus. The expression of CEF-4 is dependent on both transcriptional and post-transcriptional mechanisms of regulation. The characterization of the promoter region indicated that three distinct regulatory elements corresponding to an AP-1 binding site (or TRE), a PRDII/kappaB domain, and a CAAT box are involved in the activation by pp60(v-)src. In this report we investigate the signaling pathways controlling the expression of the TRE and PRDII domain. The expression of a dominant negative mutant of p21(ras) reduced the activity of both elements. In contrast a similar mutant of c-Raf-1 affected modestly the activation dependent on the TRE but not PRDII. The stress-activated protein kinase (SAPK)/Jun N-terminal kinase (JNK) pathway was important for the activity of PRDII and the TRE but was not markedly stimulated by pp60(v-)src. The addition of calphostin C and the inhibition of protein kinase C (PKC) diminished the accumulation of the CEF-4 mRNA and reduced the activity of a TRE-controlled promoter. Likewise, the depletion of PKC by chronic treatment with phorbol esters inhibited the activation of the TRE. Rous sarcoma virus-transformed CEF treated with calphostin C were also flatter, did not display a high degree of criss-crossing, and appeared morphologically normal. Hence PKC was important for the activation of AP-1 and the morphological transformation of CEF. The constitutive expression of CEF-4 was correlated with transformation only when dependent on the TRE. This was not true for PRDII, which was the only element required for the constitutive activation to the CEF-4 promoter in nontransformed cells treated chronically with phorbol esters.

Oncogenic Neu/ErbB-2 increases ets, AP-1, and NF-kappaB-dependent gene expression, and inhibiting ets activation blocks Neu-mediated cellular transformation

Overexpression of Neu (ErbB-2/HER2) is found in approximately 20% of breast tumors. Activation of Neu by a point mutation (NeuT) causes constitutive tyrosine kinase activity of this transmembrane receptor and transforming activity in fibroblasts. To identify downstream targets of Neu, we have analyzed the ability of Neu to activate gene expression. Expression of NeuT, but not normal Neu, caused transcriptional activation of Ets, AP-1, or NF-kappaB-dependent reporter genes. Dominant inhibitory Ras or Raf mutants blocked the Neu-mediated transcriptional activation, confirming that Ras signaling pathways were required for this activation. Analysis with Ets2 mutants indicated that activation of Ets2 transcriptional activity mediated by NeuT or oncogenic Ras required phosphorylation of the same Ets2 residue, threonine 72. Cotransfection of dominant inhibitory Ets2 mutants specifically blocked NeuT-mediated activation of Ets-dependent reporter genes. Furthermore, in focus formation assays using NIH 3T3 cells, the transforming activity of NeuT was inhibited 5-fold when NeuT was cotransfected with a dominant negative Ets2 mutant. However, parallel colony formation assays showed that the Ets2 dominant negative mutant did not inhibit the growth of normal cells. Together, these data show that NeuT activates a variety of transcription factor families via the Ras signaling pathway and that Ets activation is required for NeuT-mediated cellular transformation. Thus, downstream targets of Neu, including Ets transcription factors, may be useful points for therapeutic intervention in Neu/ErbB-2-associated cancers.

Design of a C/EBP-specific, dominant-negative bZIP protein with both inhibitory and gain-of-function properties

We have developed a bZIP protein, GBF-F, with both dominant-negative (DN) and gain-of-function properties. GBF-F is a chimera consisting of two components: the DNA binding (basic) region from the plant bZIP protein GBF-1 (GBF) and a leucine zipper (F) designed to preferentially heterodimerize with the C/EBP alpha leucine zipper. Biochemical studies show that GBF-F preferentially forms heterodimers with C/EBP alpha and thus binds a chimeric DNA sequence composed of the half-sites recognized by the C/EBP and GBF basic regions. Transient transfections in HepG2 hepatoma cells show that both components of GBF-F are necessary for inhibition of C/EBP alpha transactivation. When the C/EBP alpha leucine zipper is replaced with that of either GCN4 or VBP, the resulting protein can transactivate a C/EBP cis-element but is not inhibited by GBF-F, indicating that the specificity of dominant-negative action is determined by the leucine zipper. All known members of the C/EBP family contain similar leucine zipper regions and are inhibited by GBF-F. GBF-F also exhibits gain-of-function properties, since, with the essential cooperation of a C/EBP family member, it can transactivate a promoter containing the chimeric C/EBP/GBF site. This protein therefore has potential utility both as a dominant-negative inhibitor of C/EBP function and as an activator protein with novel DNA sequence specificity.

Fos leucine zipper variants with increased association capacity

The Fos wild-type leucine zipper is unable to support homodimerization. This finding is generally explained by the negative net charge of the Fos zipper leading to the electrostatic repulsion of two monomers. Using a LexA-dependent in vivo assay in Escherichia coli, we show here that additional antideterminants for Fos zipper association are the residues in position a within the Fos zipper interface. If the wild-type Fos zipper is fused to the DNA binding domain of the LexA repressor (LexA-DBD), no excess repression is observed as compared with the LexA-DBD alone, in agreement with the incapacity of the wild-type Fos zipper to promote homodimerization. If hydrophobic amino acids (Ile, Leu, Val, Phe, Met) are inserted into the five a positions of a LexA-Fos zipper fusion protein, substantial transcriptional repression is recovered showing that Fos zipper homodimerization is not only limited by the repulsion of negatively charged residues but also by the nonhydrophobic nature of the a positions. The most efficient variants (harboring Ile or Leu in the five a positions) show an about 80-fold increase in transcriptional repression as compared with the wild-type Fos zipper fusion protein. In the case of multiple identical substitutions, the overall improvement is correlated with the hydrophobicity of the inserted side chains, i.e. Ile Leu > Val > Phe > Met. However at least for Val, Phe, and Met the impact of a given residue type on the association efficiency depends strongly on the heptad, i.e. on the local environment of the a residue. This is particularly striking for the second heptad of the Fos zipper, where Val is less well tolerated than Phe and Met. Most likely the a1 residue modulates the interhelical repulsion between two glutamic acid side chains in positions g1 and e2. Most of the hydrophobic Fos zipper variants are also improved in heteroassociation with a Jun leucine zipper, such that roughly half of the additional free energy of homodimerization is imported into the heterodimer. A few candidates (including the Fos wild-type zipper) deviate from this correlation, showing considerable excess heteroassociation.

Aberrant function of the Ras signal transduction pathway in human breast cancer

Although ras mutations are infrequent (approximately 5%) in breast cancers, there is considerable evidence that suggests that the pathways which Ras services may still be deregulated in breast cancer cells. The recent identification of many of the components of the Ras signal transduction pathway has defined a network of proto-oncogene proteins controlling diverse signaling events that regulate cell growth and differentiation. Consequently, mutations that perturb the function of any one component of this signal pathway may trigger the same oncogenic events as mutation of ras itself. Moreover, several Ras-related proteins have recently been demonstrated to possess the ability to trigger malignant transformation via signaling pathways shared with Ras proteins. Thus, it is possible that the aberrant function of Ras-related proteins may contribute to breast cancer development. Consequently, it is important not to dismiss the Ras pathway in the development of breast cancer merely because of the infrequent detection of mutations in ras itself, but rather to consider the influence of aberrations upstream or downstream of Ras and of certain Ras-related proteins in the development of breast cancer. Finally, the critical importance of components upstream and downstream of Ras provides additional targets for rational drug design approaches to block the aberrant function of Ras signaling in human tumors.

Transcriptional repression of the alpha 1(I) collagen gene by ras is mediated in part by an intronic AP1 site

We have previously shown that transformation of fibroblasts by ras results in transcriptional inhibition of the alpha 1(I) gene. An alpha 1(I)-hGH chimeric plasmid containing 3.7 kb of 5' flanking and 4.4 kb of alpha 1(I) transcribed sequence was regulated appropriately by ras in a transient transfection assay. In contrast, a similar plasmid containing alpha 1(I) DNA from -220 to +500 was virtually unresponsive to ras. The regions from -3700 to -220 and +500 to +4400 contributed equally to the ras-mediated inhibition of the parental plasmid. Deletion analysis indicated that a short fragment, between +500 and +890 in the first intron of the alpha 1(I) gene, was recognized differently in ras-transformed and wild-type cells. A previously described AP1 site in this fragment stimulated alpha 1(I) transcription in Rat1 fibroblasts but was inactive in ras-transformed cells. Mobility shift assays using nuclear extracts from the two cell types demonstrated differences in binding to the alpha 1(I) AP1 site. We conclude that ras transformation suppresses the function of a cell-specific enhancer in the first intron of the alpha 1(I) collagen gene.

Targeting of promoters for trans activation by a carboxy-terminal domain of the NS-1 protein of the parvovirus minute virus of mice

The NS-1 gene of the parvovirus minute virus of mice (MVM) (prototype strain, MVMp) was fused in phase with the sequence coding for the DNA-binding domain of the bacterial LexA repressor. The resulting chimeric protein, LexNS-1, was tested for its transcriptional activity by using various target promoters in which multiple LexA operator sequences had been introduced. Under these conditions, NS-1 was shown to stimulate gene expression driven by the modified long terminal repeat promoters (from the retroviruses mouse mammary tumor virus and Rous sarcoma virus) and P38 promoter (from MVMp), indicating that the NS-1 protein is a potent transcriptional activator. It is noteworthy that in the absence of LexA operator-mediated targeting, the genuine mouse mammary tumor virus and Rous sarcoma virus promoters were inhibited by NS-1. Together these data strongly suggest that NS-1 contains an activating region able to induce promoters with which this protein interacts but also to repress transcription from nonrecognized promoters by a squelching mechanism similar to that described for other activators. Deletion mutant analysis led to the identification of an NS-1 domain that exhibited an activating potential comparable to that of the whole polypeptide when fused to the DNA-binding region of LexA. This domain is localized in the carboxy-terminal part of NS-1 and corresponds to one of the two regions previously found to be responsible for toxicity. These results argue for the involvement of the regulatory functions of NS-1 in the cytopathic effect of this parvovirus product.

Oncogenic Ras activates c-Jun via a separate pathway from the activation of extracellular signal-regulated kinases

c-Jun transcriptional activity is augmented by expression of oncogenic Ras and Raf proteins. This study demonstrates a direct correlation between Ras transforming activity and c-Jun activation, supporting an important role for c-Jun in transformation by Ras. Since we observed that Ras activated c-Jun transcriptional activity by increasing phosphorylation of the c-Jun activation domain at residues Ser-63/Ser-73 and that oncogenic Ras proteins activated extracellular signal-regulated protein kinases (ERK1 and ERK2) (also known as mitogen-activated protein kinases), we evaluated the possibility that ERKs were directly responsible for c-Jun activation. Coexpression of wild-type ERKs with oncogenic Ras proteins potentiated, while kinase-defective ERKs inhibited, Ras-induced transcriptional activation from the Ras-responsive element (Ets-1/AP-1) present in the NVL-3 enhancer and the serum-response element in the c-fos promoter. In contrast, coexpression of either wild-type or kinase-defective ERKs inhibited Ras and Raf activation of c-Jun transcriptional activity. Thus, although activation of both ERK and c-Jun are downstream consequences of activation of the Ras signal transduction pathway, our results suggest that Ras-induced c-Jun phosphorylation and transcriptional activation are not a direct consequence of ERK1 and ERK2 activation.

Analysis of AP-1 function in cellular transformation pathways

To understand the role of endogenous AP-1 activity in cellular transformation induced by oncogenes, we have made use of a fos mutant (supfos-1) and a jun mutant (supjun-1), either of which can function as a transdominant inhibitor of AP-1-mediated transcriptional regulation. Chicken embryo fibroblasts (CEF) infected with a series of transforming retroviruses were doubly infected with retrovirus carrying supfos-1 or supjun-1, and suppression of cellular transformation was monitored in terms of reversion to normal cellular morphology or acquisition of anchorage-dependent growth. Cellular transformation induced by several exogenously expressed transforming genes of the fos or jun family was efficiently suppressed, as expected. CEF transformed by v-src, v-yes, v-fps, c-Ha-ras, and N-terminally truncated c-raf were also induced to revert to the normal phenotype by these transdominant mutants, suggesting that functional transcription factor AP-1 activity is essential for the cellular transformation induced by these oncogenes. The suppression is not attributable to nonspecific inhibition of cellular proliferation, because CEF transformed by v-ros or v-myc were not induced to revert to the normal phenotype. We next analyzed changes in all known components of chicken AP-1 induced by v-src, c-Ha-ras, or activated c-raf transformation. The levels of both Fra-2 and c-Jun expression were elevated two- to fourfold, and hyperphosphorylation of Fra-2 was also observed. We further showed that Fra-2-c-Jun heterodimer is mainly responsible for the elevated AP-1 DNA-binding activity in these transformed cells, and we propose that this heterodimer play a crucial role in the transformation induced by these oncogenes.

Oncogene activation of human keratin 18 transcription via the Ras signal transduction pathway

Keratin 8 (K8) and keratin 18 (K18) are intermediate filament proteins normally expressed in simple epithelial tissues and persistently expressed in a wide variety of carcinomas. Ectopic expression of K8 and K18 occurs in some epidermal and murine skin carcinomas induced by chemical carcinogenesis or oncogenic ras expression. We show here that K18 is a direct target of the Ras signal transduction pathway, by demonstrating that activated Ha-Ras, as well as activated Src, Lck, or Raf, stimulates the transcription of K18. This activation is mediated by an enhancer element containing essential and closely spaced Ets and AP-1 transcription factor binding sites. Oncogene activation of K18 transcription provides a molecular explanation for the persistent and sometimes unexpected expression of K18 in such a wide variety of tumors.

Proliferative activation of quiescent Rat-1A cells by delta FosB

Fos and Jun transcription factors are induced during the normal course of the proliferative response of quiescent cells to serum or to growth factors. We have shown that delta FosB, an alternatively spliced form of FosB, is formed as rapidly as FosB in serum-stimulated Rat-1A cells. Although delta FosB lacks the C-terminal region of FosB carrying the transactivation function, constitutive expression of delta FosB transforms Rat-1A cells as does expression of FosB. The transforming ability of delta FosB suggests that delta FosB may lead to proliferative activation of quiescent cells without activating AP-1-responsive genes. To address this question, FosB or delta FosB was expressed as a fusion protein with the ligand binding domain of the human estrogen receptor (ER) in Rat-1A cells. After estrogen treatment, the fusion protein accumulates in nuclei and forms stable complexes with Jun proteins. We have shown that ER-delta FosB or to a lesser extent ER-FosB triggers quiescent Rat-1A cells to transit G1, initiate DNA replication, and ultimately undergo cell division at least once. Since ER-FosB, but not ER-delta FosB, induced expression of the AP-1-responsive transin/stromelysin gene, we concluded that the N-terminal region and the DNA binding domain of FosB or delta FosB itself have the potential to regulate cell proliferation and that the transactivation function carried by the C-terminal region of FosB is not essential for the proliferative activation of quiescent cells.

Proliferative activation of quiescent Rat-1A cells by delta FosB

Fos and Jun transcription factors are induced during the normal course of the proliferative response of quiescent cells to serum or to growth factors. We have shown that delta FosB, an alternatively spliced form of FosB, is formed as rapidly as FosB in serum-stimulated Rat-1A cells. Although delta FosB lacks the C-terminal region of FosB carrying the transactivation function, constitutive expression of delta FosB transforms Rat-1A cells as does expression of FosB. The transforming ability of delta FosB suggests that delta FosB may lead to proliferative activation of quiescent cells without activating AP-1-responsive genes. To address this question, FosB or delta FosB was expressed as a fusion protein with the ligand binding domain of the human estrogen receptor (ER) in Rat-1A cells. After estrogen treatment, the fusion protein accumulates in nuclei and forms stable complexes with Jun proteins. We have shown that ER-delta FosB or to a lesser extent ER-FosB triggers quiescent Rat-1A cells to transit G1, initiate DNA replication, and ultimately undergo cell division at least once. Since ER-FosB, but not ER-delta FosB, induced expression of the AP-1-responsive transin/stromelysin gene, we concluded that the N-terminal region and the DNA binding domain of FosB or delta FosB itself have the potential to regulate cell proliferation and that the transactivation function carried by the C-terminal region of FosB is not essential for the proliferative activation of quiescent cells.

Functional domains of the AraC protein

The AraC protein, which regulates the L-arabinose operons in Escherichia coli, was dissected into two domains that function in chimeric proteins. One provides a dimerization capability and binds the ligand arabinose, and the other provides a site-specific DNA-binding capability and activates transcription. In vivo and in vitro experiments showed that a fusion protein consisting of the N-terminal half of the AraC protein and the DNA-binding domain of the LexA repressor dimerizes, binds well to a LexA operator, and represses expression of a LexA operator-beta-galactosidase fusion gene in an arabinose-responsive manner. In vivo and in vitro experiments also showed that a fusion protein consisting of the C-terminal half of the AraC protein and the leucine zipper dimerization domain from the C/EBP transcriptional activator binds to araI and activates transcription from a PBAD promoter-beta-galactosidase fusion gene. Dimerization was necessary for occupancy and activation of the wild-type AraC binding site.