The chicken c-Jun 5' untranslated region directs translation by internal initiation.

The 5' untranslated region (UTR) of the chicken c-jun message is exceptionally GC rich and has the potential to form a complex and extremely stable secondary structure. Because stable RNA secondary structures can serve as obstacles to scanning ribosomes, their presence suggests inefficient translation or initiation through alternate mechanisms. We have examined the role of the c-jun 5' UTR with respect to its ability to influence translation both in vitro and in vivo. We find, using rabbit reticulocyte lysates, that the presence of the c-jun 5' UTR severely inhibits translation of both homologous and heterologous genes in vitro. Furthermore, translational inhibition correlates with the degree of secondary structure exhibited by the 5' UTR. Thus, in the rabbit reticulocyte lysate system, the c-jun 5' UTR likely impedes ribosome scanning resulting in inefficient translation. In contrast to our results in vitro, the c-jun 5' UTR does not inhibit translation in a variety of different cell lines suggesting that it may direct an alternate mechanism of translational initiation in vivo. To distinguish among the alternate mechanisms, we generated a series of bicistronic expression plasmids. Our results demonstrate that the downstream cistron, in the bicistronic gene, is expressed to a much higher level when directly preceded by the c-jun 5' UTR. In addition, inhibition of ribosome scanning on the bicistronic message, through insertion of a synthetic stable hairpin, inhibits translation of the first cistron but does not inhibit translation of the cistron downstream of the c-jun 5' UTR. These results are consistent with a model by which the c-jun message is translated through cap independent internal initiation. Oncogene (2000) 19, 2836 - 2845

Enhanced cell motility and invasion of chicken embryo fibroblasts in response to Jun over-expression.

Malignant tumor cells exhibit a number of distinct properties involved not only with deregulated cell proliferation but also enhanced migration and invasion. The Jun oncogene has been well studied in regard to its role in cell proliferation. Many of the target genes deregulated by Jun encode matrix metalloproteases (MMPs) such as MMP1, MMP3 and MMP9. These targets implicate a prominent role for Jun in tumor cell invasion, in addition to its role in growth transformation. To investigate this possibility, we have examined the effect of over-expression of transforming and non-transforming versions of Jun on motility and invasion of chicken embryo fibroblasts (CEFs). We found that over-expression of either form of Jun results in elevated intrinsic cellular motility as well as increased motility in response to several different chemo-attractants, including 3T3-conditioned media, basic fibroblast growth factor, hepatocyte growth factor and Matrigel. The capacity of these cells to invade through Matrigel is also elevated as a result of Jun over-expression. In addition to these effects, CEFs expressing Jun secrete factors that stimulate the motility of a human tongue carcinoma cell line. Our results suggest that Jun plays an important role in the potentiation of cell motility and invasion through multiple mechanisms.

Apolipoprotein A-1 is a negative target of v-Jun overexpression.

The product of the Jun oncogene influences a variety of processes including cell proliferation and differentiation. Jun exerts its influence by binding to the promoter and enhancer regions of a number of different target genes resulting in their activation or repression. We describe here the isolation and characterization of a gene differentially downregulated upon overexpression of v-Jun but not c-Jun. DNA and amino acid homology search analysis revealed this gene to be identical to chicken apolipoprotein A-1, the major component of high density lipoprotein (HDL). The half life of apolipoprotein A-1 RNA remains constant in the presence or absence of v-Jun overexpression suggesting downregulation by v-Jun is at the level of promoter activity. Consistent with this hypothesis, apolipoprotein A-1 upstream promoter fragments active in normal and c-Jun expressing CEF are inactive in v-Jun transformed CEF. Analysis of expression of apolipoprotein A-1 in CEF overexpressing other oncogenes revealed a similar downregulation by Myc and v-Src but not c-Fos, v-Ha-Ras, c-Src or c-Ski. Our findings point to a potential regulatory affect on cholesterol metabolism by v-Jun, as a result of altered levels of apolipoprotein A-1 message expression.

Isolation and cloning of JTAP-1: a cathepsin like gene upregulated in response to V-Jun induced cell transformation.

The oncogenic potential of Jun in chicken embryo fibroblasts (CEF) varies depending on its structure. V-Jun, which has a number of structural differences from c-Jun is highly transforming and tumorigenic. C-Jun however, is only weakly transforming and is not tumorigenic. We have used this difference in oncogenic potential between v-Jun and c-Jun to screen for downstream target genes associated with the v-Jun induced transformed phenotype. We describe here the identification, cloning and characterization of one of these genes, JTAP-1. JTAP-1 is consistently overexpressed 7 to 10-fold in CEF transformed by v-Jun compared with c-Jun overexpressing or normal CEF. This pattern of expression suggests that JTAP-1 is associated with the transformed phenotype. DNA and amino acid homology search analysis revealed that JTAP-1 shares a high degree of similarity with over 100 cysteine proteases from a variety of species and is likely the chicken homolog of cathepsin O. Analysis of expression of JTAP-1 in CEF overexpressing other oncogenes including v-Ha-ras, v-Src, c-Fos, and Myc revealed that it's overexpression is unique to v-Jun transformed cells. Thus, JTAP-1 is likely a specific target of v-Jun overexpression and not simply a consequence of cell transformation.

In vivo viral and cellular Jun complexes exhibit differential interaction with a number of in vitro generated 'AP-1- and CREB-like' target sequences.

A direct comparison of the relative DNA-binding capabilities of in vivo Jun-containing complexes derived from overexpression of the highly transforming viral Jun (VJ-1 CEF), the weakly transforming chicken cellular Jun (CJ-3 CEF) or background endogenous Jun (RCAS CEF) was assessed by gel mobility-shift assays using a synthetic oligonucleotide containing the consensus sequence TGACTCA (consensus AP-1). Chicken embryo fibroblasts (CEFs) expressing background c-Jun levels (RCAS CEF) contain almost undetectable levels of c-Jun but retain significant DNA-binding activity with two distinct complexes capable of binding specifically to the consensus AP-1 site. CEFs overexpressing either v-Jun or c-Jun contain these same two complexes and, while showing marked increases in Jun protein levels, do not exhibit any increase in DNA binding or transcriptional activation activity, suggesting that much of the overexpressed protein is inactive. Gel-shift assays performed in the presence of a Jun-specific antibody revealed a reduction in binding by both complexes, suggesting that each contains Jun or a Jun cross-reactive protein. Antibodies specific for Jun B, c-Fos, Fos B and CREB failed to interact with either complex. However, antibody specific for Fra-2 caused a slight supershift, suggesting that one or both complexes may contain Fra-2. Gel-shift competition assays with 16 'AP-1- and CREB-like' target sequences revealed that, within each cell type, the two protein complexes varied in their ability to recognize the mutant target sequences. These results clearly indicate differences in potential target recognition by each specific in vivo complex, and suggest that each may preferentially bind its own subset of target DNAs. In addition, a comparison of binding by individual complexes derived from CEFs overexpressing v-Jun and c-Jun also revealed differences in target recognition. Thus, in vivo complexes formed by overexpression of v-Jun and c-Jun vary in their ability to recognize and bind to a number of 'AP-1- and CREB-like' target sequences. This has important implications with regard to the mechanisms involved in cell transformation by v-Jun.

Transformation by Jun: requirement for leucine zipper, basic region and transactivation domain and enhancement by Fos.

Mutants in the leucine zipper and basic regions of mouse c-jun were tested for transformation in chicken embryo fibroblast cultures. Reduction or elimination of the ability of Jun to dimerize or to bind to DNA severely decreased transformation. A chicken v-jun gene from which the major transactivation domain was deleted also failed to transform. We conclude that an intact leucine zipper, basic region and transactivation domain are required for Jun-induced oncogenic transformation. Coexpression of chicken c-Fos increased formation of transformed foci by Jun proteins of moderate to low oncogenic potency but had no effect on highly transforming Jun. Chicken c-Fos could also transform chicken embryo fibroblasts on its own, albeit after prolonged culture and at a low efficiency.

Oncogenes and cell growth.

In this short overview of oncogenes and cell growth, the protein products have been divided into two classes, proto-oncogenes and oncogenes. Proto-oncogenes can be activated by point mutations and deletions. Two classes exist: the dominant, which leads to cell growth and the suppressor, which by definition suppresses growth. The mechanism of action is multiplex--duplication of hormone action, resemblance to receptors, or kinases and DNA binding proteins. It is clear that the regulation of cell growth and differentiation is very complex and that the products of proto-oncogenes play important roles in this regulation. Their functions appear to be at two levels. The first level is that of transduction of signals to the nucleus where the signals can be acted upon. The second is at the level of specific gene regulation, where incoming signals are turned into a response by the cell through activation of specific genetic programs. Nuclear proto-oncogene products play intimate roles in activation of these programs. The nature of the specific target genes regulated in response to these oncogene and proto-oncogene products however, remains a critical area of intensive research.

Jun inhibits myogenic differentiation.

Myoblasts from skeletal muscle of chicken or Japanese quail embryos were infected with avian sarcoma virus 17 (ASV-17), a retrovirus carrying the jun oncogene. At high multiplicities of infection ASV-17-induced morphologic transformation inhibited fusion of myoblasts into myotubes and stimulated extended replication. The expression of the muscle-specific proteins desmin, myosin and creatine phosphokinase was inhibited in ASV-17-infected cultures. Immunofluorescent staining detected strong expression of the ASV-17 Gag-Jun fusion protein in the nuclei of infected mononuclear myoblasts, but Gag-Jun was not detectable in multinucleated myotubes that occurred in clonal populations of ASV-17-infected quail myoblasts. This result suggests that the nuclear expression of viral jun and myogenic differentiation are mutually exclusive events. A mutant of ASV-17, ts jun-1, is partly temperature-sensitive in its ability to transform chicken embryo fibroblasts. At the non-permissive temperature of 41.5 degrees C, multinucleated myotubes readily formed in ts jun-1-infected myoblast cultures and expressed muscle-specific proteins detectable by immunofluorescent staining. These myotubes also showed strong immunofluorescent staining for Gag-Jun in the cell nuclei. The nuclear expression of a Jun protein that is defective in its transforming function appears therefore to be compatible with myogenesis. Several retroviral constructs carrying various viral and cellular jun inserts, as well as jun deletion mutants and recombinants between c-jun and v-jun, were tested for their effect on myogenic differentiation. There was an approximate correlation between the ability of a construct to transform chicken embryo fibroblasts and its effectiveness in interfering with myogenic differentiation. We conclude that the expression of an oncogenic jun gene in myoblasts strongly inhibits myogenic differentiation, and that a highly transforming Jun protein cannot be expressed in the nuclei of differentiating myotubes, while the presence of transformation-defective variants of Jun is compatible with differentiation.

Efficient transformation of chicken embryo fibroblasts by c-Jun requires structural modification in coding and noncoding sequences.

To assess the transforming capability of the c-Jun protein, we introduced the chicken c-jun proto-oncogene into a replication competent avian retroviral expression vector (RCAS). Viral Jun efficiently transformed chicken embryo fibroblasts (CEFs) when expressed from this vector. Overexpression of c-Jun leads to transformation of CEFs with an efficiency that is 15- to 25-fold less than that seen for v-Jun, suggesting that v-Jun contains structural features that increase its oncogenic potential relative to c-Jun. There are four structural differences between v-Jun and c-Jun. To determine the relative contribution that each of these structural differences between v-Jun and c-Jun has on oncogenic activity, several deletion and substitution mutants were constructed. Each of these mutants was expressed in CEF and assayed for transformation by focus formation. Analysis of the results reveals that deletion of a region of 27 amino acids near the amino terminus of c-Jun and deletion of 3'-untranslated sequences are critical in activating the full oncogenic potential of Jun.

The oncogene jun and nuclear signalling.

Jun is a transcription factor that can also induce oncogenic transformation. Its DNA-binding domain is conserved from yeast to man and shows homology to several other transcriptional regulators. Jun dimerizes with the fos protein through an alpha-helical domain termed the leucine zipper, and the jun-fos heterodimers bind to DNA and regulate transcription of numerous specific unlinked genes.

Enhanced jun gene expression is an early genomic response to transforming growth factor beta stimulation.

Transforming growth factor beta (TGF beta) is a multifunctional polypeptide that regulates proliferation, differentiation, and other functions of many cell types. The pathway of TGF beta signal transduction in cells is unknown. We report here that an early effect of TGF beta is an enhancement of the expression of two genes encoding serum- and phorbol ester tumor promoter-regulated transcription factors: the junB gene and the c-jun proto-oncogene, respectively. This stimulation was observed in human lung adenocarcinoma A549 cells which were growth inhibited by TGF beta, AKR-2B mouse embryo fibroblasts which were growth stimulated by TGF beta, and K562 human erythroleukemia cells, which were not appreciably affected in their growth by TGF beta. The increase in jun mRNA occurred with picomolar TGF beta concentrations within 1 h of TGF beta stimulation, reached a peak between 1 and 5 h in different cells, and declined gradually to base-line levels. This mRNA response was followed by a large increase in the biosynthesis of the c-jun protein (AP-1), as shown by metabolic labeling and immunoprecipitation analysis. However, differential and cell type-specific regulation appeared to determine the timing and magnitude of the response of each jun gene in a given cell. In AKR-2B and NIH 3T3 cells, only junB was induced by TGF beta, evidently in a protein synthesis-independent fashion. The junB response to TGF beta was maintained in c-Ha-ras and neu oncogene-transformed cells. Thus, one of the earliest genomic responses to TGF beta may involve nuclear signal transduction and amplification by the junB and c-jun transcription factors in concert with c-fos, which is also induced. The differential activation of the jun genes may explain some of the pleiotropic effects of TGF beta.

The carboxy terminus of the viral Jun oncoprotein is required for complex formation with the cellular Fos protein.

The products of the proto-oncogenes c-jun and c-fos are known to form a complex in vivo. Complex formation appears to stabilize protein-DNA interactions and is thought to play an important functional role in transcriptional regulation. Here we show that the viral Jun oncoprotein, which differs structurally from cellular Jun, is also capable of complex formation with Fos. Thus the oncogenic potency of viral Jun is unlikely to be due to an altered affinity for Fos. We have also defined, by deletion analysis, the domain of v-Jun responsible for complex formation to reside in the carboxy terminus encompassing the leucine zipper motif. We find that complex formation with c-Fos does not occur with v-Jun deletions affecting one or more leucine residues in the zipper domain. Our results are consistent with the hypothesis that the leucine zipper mediates Jun-Fos interaction.

The oncogenicity of jun.

The oncogenic transforming potential of the jun oncogene was investigated with viral constructs that contain various terminal deletions of v-jun, c-jun and recombinants between the viral and cellular genes. Cellular jun can induce transformation of chicken embryo fibroblasts with a low efficiency. High efficiency transformation is dependent on alterations in the major transactivator domain. A jun deletion that lacks the transactivator domain is non-transforming. These observations are compatible with the conclusion that transcriptional regulation plays an important part in jun oncogenesis. In cells transformed by viral jun and expressing high levels of viral jun expression of the cellular jun is not constitutively upregulated. Cellular jun may therefore not contribute to transformation in these cells.

Interaction of cellular factors related to the Jun oncoprotein with the promoter of a replication-dependent hamster histone H3.2 gene.

The promoter region of a replication-dependent histone H3.2 gene contains a putative DNA binding site for the Jun oncoprotein within a 32-nucleotide regulatory domain. The hamster sequence differs by one nucleotide from the AP-1 consensus sequence found in several promoters responsive to phorbol 12-myristate 13-acetate. We have identified the factors interacting with this region as 42- and 45-kDa proteins by DNA affinity chromatography, immunoblotting, and UV crosslinking. These proteins, which are candidates for conferring high-level expression to the histone promoter, share an antigenic epitope with the DNA-binding domain of Jun but diverge from it at the amino terminus. The interaction of these proteins with the promoter of a replication-dependent cellular gene provides evidence that members of the Jun oncoprotein family may play specific roles in the expression of genes essential for progression of the cell cycle.

Fos-associated protein p39 is the product of the jun proto-oncogene.

The Fos protein complex and several Fos-related antigens (FRA) bind specifically to a sequence element referred to as the HeLa cell activator protein 1 (AP-1) binding site. A combination of structural and immunological comparisons has identified the Fos-associated protein (p39) as the protein product of the jun proto-oncogene (c-Jun). The p39/Jun protein is one of the major polypeptides identified in AP-1 oligonucleotide affinity chromatography extracts of cellular proteins. These preparations of AP-1 also contain Fos and several FRA's. Some of these proteins bind to the AP-1 site directly whereas others, like Fos, appear to bind indirectly via protein-protein interactions. Cell-surface stimulation results in an increase in c-fos and c-jun products. Thus, the products of two protooncogenes (and several related proteins), induced by extracellular stimuli, form a complex that associates with transcriptional control elements containing AP-1 sites, thereby potentially mediating the long-term responses to signals that regulate growth control and development.

v-jun encodes a nuclear protein with enhancer binding properties of AP-1.

The jun oncogene of ASV17 is expressed as a 65 kd protein (p65gag-jun) that contains partial gag sequences at its amino terminus fused to jun sequences that make up the carboxy terminal two-thirds of the molecule. As a first step toward evaluating potential functional differences between the activated oncogene, v-jun, and its cellular counterpart, c-jun, we have characterized the biochemical properties of the gag-jun product of ASV17. Immunofluorescence studies revealed that the v-jun protein is localized in the nucleus of CEF transfected with ASV17 DNA. DNAase I foot-printing analysis indicates that p65gag-jun synthesized in bacteria binds to enhancer elements in SV40 that are recognition sites for the human transcription factor AP-1. Analysis of point mutants confirmed that v-jun protein binds with DNA sequence specificity of the mammalian enhancer factor AP-1 and the yeast transcription factor GCN4. These findings suggest that activation of the jun oncogene may not exclusively be the result of alterations in the DNA binding properties of the normal cellular protein.