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

The v-Jun point mutation allows c-Jun to escape GSK3-dependent recognition and destruction by the Fbw7 ubiquitin ligase

The c-Jun and c-Myc oncogenic transcription factors are highly unstable proteins due to polyubiquitination. Similar to c-Myc, we report here that phosphorylation of c-Jun by GSK3 creates a high-affinity binding site for the E3 ligase Fbw7, which targets c-Jun for polyubiquitination and proteasomal degradation. In keeping with this, we found that c-Jun levels were inversely related to GSK3 activity in mammalian cells that had entered the cell cycle. Importantly, phosphorylation of c-Jun by GSK3 requires a priming phosphorylation event at Ser-243. Ser-243 is mutated to phenylalanine in v-Jun and allows it to escape recognition by Fbw7. These findings explain the enhanced stability and oncogenicity of v-Jun relative to its cellular counterpart and reveal that GSK3 and Fbw7 coordinately regulate c-Jun and c-Myc.

v-Jun downregulates the alpha 2 (I) collagen target gene indirectly through Sp1/3

Transformation of chick embryo fibroblasts (CEFs) by the v-Jun oncoprotein correlates with a downregulation of the alpha 2 (I) collagen gene. To investigate whether this gene constitutes a direct target of v-Jun, an analysis of a large proximal fragment of the promoter, extending from position -1080 to +109, was performed. Transient transfections with -1080/+109 and deleted derivatives revealed that a short proximal fragment, -433/+11, is the target for repression by v-Jun. Extensive analysis, conducted in CEFs and in Sp1/3-deficient Drosophila SL2 cells, further showed that (i) high constitutive activity of -433/+11 requires a direct binding of the ubiquitous Sp1 and/or Sp3 transcription factors acting on two distinct motifs, that is, a proximal TCC-rich region and an upstream GC box, and that (ii) repression by v-Jun does not require any direct binding of the oncoprotein to the DNA, but an indirect binding within a v-Jun-Sp1/3-DNA chromatin-associated complex. This situation is reminiscent of a situation previously reported with the tata-less, SPARC (secreted protein, acidic, and rich in cysteine) target promoter that regulates the expression of another extracellular matrix component in the same model of cell transformation. Taken together, these data reinforce the view that, at least in CEFs, v-Jun downregulates a family of direct target genes by binding to the DNA indirectly through Sp1/3.

v-Jun downregulates the SPARC target gene by binding to the proximal promoter indirectly through Sp1/3

Transformation of chick embryo fibroblasts by the v-Jun oncoprotein correlates with a downregulation of the extracellular matrix protein SPARC and repression of the corresponding mRNA. Repression of SPARC contributes to the oncogenic process by facilitating tumor development in vivo. A proximal promoter fragment, designated -124/+16, is responsible for high constitutive activity of the SPARC gene and is the target of repression by v-Jun. In this paper, using electrophoretic mobility shift and pull-down assays in vitro, and transient transfections and chromatin immunoprecipitation assays in Sp1/3-deficient Drosophila SL2 cells and in chick embryo fibroblasts, we show that (i) Sp1 and/or Sp3 is required for constitutive activation of SPARC transcription, by binding directly to the GGA-rich -92/-57 fragment; and (ii) v-Jun does not bind -124/+16 directly, but binds to the GGA-rich fragment indirectly, most likely through a physical interaction with Sp1/3. Moreover, a transactivation-proficient v-Jun derivative, designated v-Jun/cebp/glz, which cannot bind Jun DNA motifs anymore and cannot heterodimerize, is still capable of downregulating SPARC efficiently. Taken together, these data strongly suggest that v-Jun downregulates SPARC through the formation of a DNA-Sp1/3-v-Jun, chromatin-associated complex.

Transcriptional upregulation of SPARC, in response to c-Jun overexpression, contributes to increased motility and invasion of MCF7 breast cancer cells

Overexpression of the c-Jun proto-oncogene in MCF7 breast cancer cells results in a variety of phenotype changes related to malignant progression including increased motility and invasion. Concurrent with these phenotypic effects are changes in the expression of multiple gene targets. We previously demonstrated that expression of the SPARC/osteonectin gene, while undetectable in the MCF7 cell line, is highly induced in response to stable c-Jun overexpression (c-Jun/MCF7). Because the SPARC gene product is associated with tumor cell invasion in a variety of different cancers, we have examined its role in mediating the phenotypic changes induced by c-Jun in MCF7 cells. We found that antisense mediated suppression of SPARC dramatically inhibits both motility and invasion in this c-Jun/MCF7 model. In contrast, stable overexpression of SPARC in the parental MCF7 cell line is not sufficient to stimulate cell motility or invasion. Examination of the promoter region of the human SPARC gene reveals three non-canonical AP-1 sites. We demonstrate that one of these sites binds c-Jun/Fra1 heterodimers in vitro, but that this and the other AP-1 like sites are dispensable with respect to c-Jun stimulated SPARC promoter activation. Deletion analysis identified a region between -120 and -70 as a c-Jun responsive element sufficient to induce maximal promoter activation. This region does not contain any AP-1 sites but does mediate binding by SP1 'like' complexes. Furthermore, this region is necessary for SP1/SP3 responsiveness in Drosophila SL2 cells. These results demonstrate that SPARC plays an important role in stimulating motility and the invasive behavior of c-Jun/MCF7 cells and that SPARC promoter activation by c-Jun appears to occur through an indirect mechanism.

Jun, the oncoprotein

Cellular Jun (c-Jun) and viral Jun (v-Jun) can induce oncogenic transformation. For this activity, c-Jun requires an upstream signal, delivered by the Jun N-terminal kinase (JNK). v-Jun does not interact with JNK; it is autonomous and constitutively active. v-Jun and c-Jun address overlapping but not identical sets of genes. Whether all genes essential for transformation reside within the overlap of the v-Jun and c-Jun target spectra remains to be determined. The search for transformation-relevant targets of Jun is moving into a new stage with the application of DNA microarrays technology. Genetic screens and functional tests remain a necessity for the identification of genes that control the oncogenic phenotype.