The transcription factor AP-1 plays key roles in tumorigenesis, by regulating a variety of protein-coding genes, implicated in multiple hallmarks of cancer. Among non-coding genes, no AP-1 target has been described yet in tumorigenesis. MicroRNAs (miRNAs) are negative post-transcriptional regulators of protein-coding genes. miRNA expression signatures are highly relevant in cancer and several tumor-associated miRNAs (oncomirs) play critical roles in oncogenesis. Here, we show that the miRNA miR-21, which represents the most frequently upregulated oncomir in solid tumors, is induced by AP-1 in response to RAS. By analyzing validated miR-21 targets, we have found that the tumor suppressors PTEN and PDCD4 are downregulated by RAS in an AP-1-and miR-21-dependent fashion. We further show that, given the role of PDCD4 as negative regulator of AP-1, the miR-21-mediated downregulation of PDCD4 is essential for the maximal induction of AP-1 activity in response to RAS. Our data reveal a novel mechanism of positive autoregulation of the AP-1 complex in RAS transformation and disclose the function of oncomirs as critical targets and regulators of AP-1 in tumorigenesis.

We have investigated the role of the NFkB complex in the process of thyroid carcinogenesis by analysing thyroid carcinoma cell lines. A signi®cant increase in p65 NFkB mRNA and protein expression, compared to normal thyroid cultures or tissue, was found in all of the cancer cell lines. Conversely, only a modest increase in the p50 NFkB mRNA and protein was found in most, but not all carcinoma cell lines. The block of p65 protein synthesis with speci®c antisense oligonucleotides greatly reduced the ability of two undierentiated carcinoma cell lines to form colonies in agar and reduced their growth rate. On the other hand, no eect was observed in the same cell lines when treated with p50 speci®c antisense oligonucleotides. These inhibitory eects seem to be mediated by the suppression of c-myc gene expression, since treatment with antisense oligonucleotides for p65 gene interfered negatively with c-myc gene expression. Our results indicate that activation of the NFkB complex by overexpression of p65 plays a critical role in the process of thyroid cell transformation.

The expression of the high mobility group I (HMGI)-C chromatin component was shown previously to be essential for the establishment of the neoplastic phenotype in retrovirally transformed thyroid cell lines. To identify possible targets of the HMGI-C gene product, we have analyzed the AP-1 complex in normal, fully transformed and antisense HMGI-C-expressing rat thyroid cells. We show that neoplastic transformation is associated with a drastic increase in AP-1 activity, which reflects multiple compositional changes. The strongest effect is represented by the dramatic junB and fra-1 gene induction, which is prevented in cell lines expressing the antisense HMGI-C. These results indicate that the HMGI-C gene product is essential for the junB and fra-1 transcriptional induction associated with neoplastic transformation. The inhibition of Fra-1 protein synthesis by stable transfection with a fra-1 antisense RNA vector significantly reduces the malignant phenotype of the transformed thyroid cells, indicating a pivotal role for the fra-1 gene product in the process of cellular transformation.

No abstract

The AP-1 transcription factor plays an essential role in cell proliferation and tumorigenesis. It was previously shown that the fra-1 gene product is upregulated by various oncogenes and is involved in the in vitro and in vivo transformation of thyroid cells. Here we show that the ras oncogene-dependent accumulation of Fra-1 is mediated by a positive feedback mechanism which requires both transcriptional autoregulation and posttranslational stabilization of the protein. The oncogene-dependent transcriptional activation involves the cooperation between both Raf-dependent and Raf-independent pathways and is mediated by an AP-1 site within the fra-1 first intron, which becomes stably occupied by a transcriptionally active Fra-1-containing complex in ras-transformed cells. The posttranslational stabilization results in a drastic increase in the Fra-1 half-life in ras-transformed cells and is totally dependent on the activity of the MEK/ERK phosphorylation pathway. The analysis of the Fra-1 transactivation potential shows that the protein is able to stimulate a heterologous promoter in a ras-dependent manner, but the transactivating activity requires the recruitment of a heterodimeric partner. These data show that the alteration of multiple regulatory mechanisms is required for the constitutive activation of Fra-1 as a nuclear target of ras transformation.During the course of oncogenesis, the establishment of the transformed and tumorigenic phenotype requires the activity of multiple nuclear targets of transformation, including the activator protein 1 (AP-1) transcription factor, which mediates a wide range of the effects of activated oncogenes on cell growth, apoptosis, and differentiation.The AP-1 complex is formed by homo-and heterodimerization between Jun (c-Jun, JunB, and JunD), Fos (c-Fos, FosB, Fra-1, and Fra-2), and several ATF family members, all belonging to the bZIP protein superfamily, which is characterized by the basic DNA-interacting region and the leucine zipper dimerization domain (50). Given the combinatorial diversity resulting in a large number of distinct dimers along with the multiple control levels of expression and posttranslational modification of each component, the regulation of AP-1 activity is very complex, allowing the performance of highly disparate biological functions by a single transcription factor complex (reviewed in references 5, 25, 27, 32, and 51).In normal cells, AP-1 is involved in the control of cell proliferation and, depending on the cellular context, can have both proapoptotic and antiapoptotic functions in response to a wide range of environmental stimuli, such as mitogens, stressinducing agents, inflammatory cytokines, etc. (50).Recently, important information was provided on the role of Jun proteins in the control of cell cycle, showing that both the positive effect of c-Jun and the negative control by JunB are mediated by their dual effects on the expression of cyclin D1 (6, 62) and the p16 and p21 cdk inhibitors (41, 47), while the role of JunD appears to be cell con...