Chemoprevention refers to the use of agents to inhibit, reverse or retard tumorigenesis. Numerous phytochemicals derived from edible plants have been reported to interfere with a specific stage of the carcinogenic process. Many mechanisms have been shown to account for the anticarcinogenic actions of dietary constituents, but attention has recently been focused on intracellular-signalling cascades as common molecular targets for various chemopreventive phytochemicals.

Cancer chemoprevention with dietary phytochemicals

MicroRNAs (miRNAs) are small noncoding RNAs that typically inhibit the translation and stability of messenger RNAs (mRNAs), controlling genes involved in cellular processes such as inflammation, cell-cycle regulation, stress response, differentiation, apoptosis, and migration. Thus, miRNAs have been implicated in the regulation of virtually all signaling circuits within a cell, and their dysregulation has been shown to play an essential role in the development and progression of cancer. Here, after a brief description of miRNA genomics, biogenesis, and function, we discuss the effects of miRNA dysregulation in the cellular pathways that lead to the progressive conversion of normal cells into cancer cells and the potential to develop new molecular miRNA-targeted therapies.

MicroRNAs in cancer.

Apoptosis is a physiological process critical for organ development, tissue homeostasis, and elimination of defective or potentially dangerous cells in complex organisms. Apoptosis can be initiated by a wide variety of stimuli, which activate a cell suicide program that is constitutively present in most vertebrate cells. In diverse cell types, Rel/NF-kappaB transcription factors have been shown to have a role in regulating the apoptotic program, either as essential for the induction of apoptosis or, perhaps more commonly, as blockers of apoptosis. Whether Rel/NF-kappaB promotes or inhibits apoptosis appears to depend on the specific cell type and the type of inducer. An understanding of the role of Rel/NF-kappaB transcription factors in controlling apoptosis may lead to the development of therapeutics for a wide variety of human diseases, including neurodegenerative and immune diseases, and cancer.

Control of apoptosis by Rel/NF-kappaB transcription factors.

Rel/NF-kappaB transcription factors are key regulators of immune, inflammatory and acute phase responses and are also implicated in the control of cell proliferation and apoptosis. Remarkable progress has been made in understanding the signal transduction pathways that lead to the activation of Rel/NF-kappaB factors and the consequent induction of gene expression. Evidence linking deregulated Rel/NF-kappaB activity to oncogenesis in mammalian systems has emerged in recent years, consistent with the acute oncogenicity of the viral oncoprotein v-Rel in animal models. Chromosomal amplification, overexpression and rearrangement of genes coding for Rel/NF-kappaB factors have been noted in many human hematopoietic and solid tumors. Persistent nuclear NF-kappaB activity was also described in several human cancer cell types, as a result of constitutive activation of upstream signaling kinases or mutations inactivating inhibitory IkappaB subunits. Studies point to a correlation between the activation of cellular gene expression by Rel/NF-kappaB factors and their participation in the malignant process. Experiments implicating NF-kappaB in the control of the apoptotic response also support a role in oncogenesis and in the resistance of tumor cells to chemotherapy. This review focuses on the status of the rel, nfkb and ikb genes and their activity in human tumors and their association with the onset or progression of malignancies.

Aberrant rel/nfkb genes and activity in human cancer.

Thyroid cancer is a common endocrine malignancy. There has been exciting progress in understanding its molecular pathogenesis in recent years, as best exemplified by the elucidation of the fundamental role of several major signalling pathways and related molecular derangements. Central to these mechanisms are the genetic and epigenetic alterations in these pathways, such as mutation, gene copy-number gain and aberrant gene methylation. Many of these molecular alterations represent novel diagnostic and prognostic molecular markers and therapeutic targets for thyroid cancer, which provide unprecedented opportunities for further research and clinical development of novel treatment strategies for this cancer.

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Molecular pathogenesis and mechanisms of thyroid cancer

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.

An autoregulatory loop mediated by miR-21 and PDCD4 controls the AP-1 activity in RAS transformation.

We have investigated the role of the NFκB complex in the process of thyroid carcinogenesis by analysing thyroid carcinoma cell lines. A significant increase in p65 NFκB 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 NFκB mRNA and protein was found in most, but not all carcinoma cell lines. The block of p65 protein synthesis with specific antisense oligonucleotides greatly reduced the ability of two undifferentiated carcinoma cell lines to form colonies in agar and reduced their growth rate. On the other hand, no effect was observed in the same cell lines when treated with p50 specific antisense oligonucleotides. These inhibitory effects 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 NFκB complex by overexpression of p65 plays a critical role in the process of thyroid cell transformation.

Expression of the neoplastic phenotype by human thyroid carcinoma cell lines requires NFkappaB p65 protein expression.

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.

/pdf/neoplastic-transformation-of-rat-thyroid-cells-requires-the-e39use2mz2.pdf

Neoplastic transformation of rat thyroid cells requires the junB and fra‐1 gene induction which is dependent on the HMGI‐C gene product

Multi-gene families of transcription factors pose a formidable challenge to molecular and functional analysis. Dissecting distinct functions for individual family members requires a combination of approaches in different cellular and animal models. The AP-1 transcription factor complex serves as a paradigm for understanding the dynamics of transcriptional regulation. Knockout, knockdown and transgenic strategies, inducible alleles, mutational analysis, chemical genetics, etc.; researchers have applied all the tricks of the trade to understand how AP-1 works. AP-1 refers to a mixture of dimers formed between members of the Jun, Fos and ATF families. The complexity of the AP-1 biological functions reflects the wide combinatorial diversity of its components.1 AP-1 has been linked to cancer and neoplastic transformation ever since the first jun and fos genes were cloned as cellular homologues of viral oncogenes twenty years ago. Because of the oncogenic or tumor suppressive activity exhibited by distinct Jun ...

Deciphering AP-1 Function in Tumorigenesis : Fra-Ternizing on Target Promoters

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.

Laura Casalino

Papers

An autoregulatory loop mediated by miR-21 and PDCD4 controls the AP-1 activity in RAS transformation.

Expression of the neoplastic phenotype by human thyroid carcinoma cell lines requires NFkappaB p65 protein expression.

Neoplastic transformation of rat thyroid cells requires the junB and fra‐1 gene induction which is dependent on the HMGI‐C gene product

Deciphering AP-1 Function in Tumorigenesis : Fra-Ternizing on Target Promoters

Accumulation of Fra-1 in ras-Transformed Cells Depends on Both Transcriptional Autoregulation and MEK-Dependent Posttranslational Stabilization