The Ddx5 and Ddx17 RNA helicases are cornerstones in the complex regulatory array of steroid hormone-signaling pathways.

Estrogen and androgen receptors (ER and AR) play key roles in breast and prostate cancers, respectively, where they regulate the transcription of large arrays of genes. The activities of ER and AR are controlled by large networks of protein kinases and transcriptional coregulators, including Ddx5 and its highly related paralog Ddx17. The Ddx5 and Ddx17 RNA helicases are also splicing regulators. Here, we report that Ddx5 and Ddx17 are master regulators of the estrogen- and androgen-signaling pathways by controlling transcription and splicing both upstream and downstream of the receptors. First, Ddx5 and Ddx17 are required downstream of ER and AR for the transcriptional and splicing regulation of a large number of steroid hormone target genes. Second, Ddx5 and Ddx17 act upstream of ER and AR by controlling the expression, at the splicing level, of several key regulators of ER and AR activities. Of particular interest, we demonstrate that Ddx5 and Ddx17 control alternative splicing of the GSK3ß kinase, which impacts on both ER and AR protein stability. We also provide a freely available online resource which gives information regarding splicing variants of genes involved in the estrogen- and androgen-signaling pathways.

Splicing factor and exon profiling across human tissues.

It has been shown that alternative splicing is especially prevalent in brain and testis when compared to other tissues. To test whether there is a specific propensity of these tissues to generate splicing variants, we used a single source of high-density microarray data to perform both splicing factor and exon expression profiling across 11 normal human tissues. Paired comparisons between tissues and an original exon-based statistical group analysis demonstrated after extensive RT-PCR validation that the cerebellum, testis, and spleen had the largest proportion of differentially expressed alternative exons. Variations at the exon level correlated with a larger number of splicing factors being expressed at a high level in the cerebellum, testis and spleen than in other tissues. However, this splicing factor expression profile was similar to a more global gene expression pattern as a larger number of genes had a high expression level in the cerebellum, testis and spleen. In addition to providing a unique resource on expression profiling of alternative splicing variants and splicing factors across human tissues, this study demonstrates that the higher prevalence of alternative splicing in a subset of tissues originates from the larger number of genes, including splicing factors, being expressed than in other tissues.

Regulation of H-ras splice variant expression by cross talk between the p53 and nonsense-mediated mRNA decay pathways.

When cells are exposed to a genotoxic stress, a DNA surveillance pathway that involves p53 is activated, allowing DNA repair. Eukaryotic cells have also evolved a mechanism called mRNA surveillance that controls the quality of mRNAs. Indeed, mutant mRNAs carrying premature translation termination codons (PTCs) are selectively degraded by the nonsense-mediated mRNA decay (NMD) pathway. However, in the case of particular genes, such as proto-oncogenes, mutations that do not create PTCs and therefore that do not induce mRNA degradation, can be harmful to cells. In this study, we showed that the H-ras gene in the absence of mutations produces an NMD-target splice variant that is degraded in the cytosol. We observed that a treatment with the genotoxic stress inducer camptothecin for 6 h favored the production of the H-ras NMD-target transcript degraded in the cytosol by the NMD process. Our data indicated that the NMD process allowed the elimination of transcripts produced in response to a short-term treatment with camptothecin from the major proto-oncogene H-ras, independently of PTCs induced by mutations. The camptothecin effects on H-ras gene expression were p53 dependent and involved in part modulation of the SC35 splicing factor. Interestingly, a long-term treatment with camptothecin as well as p53 overexpression for 24 h resulted in the accumulation of the H-ras NMD target in the cytosol, although the NMD process was not completely inhibited as other NMD targets are not stabilized. Finally, Upf1, a major NMD effector, was necessary for optimal p53 activation by camptothecin, which is consistent with recent data showing that NMD effectors are required for genome stability. In conclusion, we identified cross talk between the p53 and NMD pathways that regulates the expression levels of H-ras splice variants.

A recently evolved class of alternative 3'-terminal exons involved in cell cycle regulation by topoisomerase inhibitors.

Alternative 3'-terminal exons, which use intronic polyadenylation sites, are generally less conserved and expressed at lower levels than the last exon of genes. Here we discover a class of human genes, in which the last exon appeared recently during evolution, and the major gene product uses an alternative 3'-terminal exon corresponding to the ancestral last exon of the gene. This novel class of alternative 3'-terminal exons are downregulated on a large scale by doxorubicin, a cytostatic drug targeting topoisomerase II, and play a role in cell cycle regulation, including centromere-kinetochore assembly. The RNA-binding protein HuR/ELAVL1 is a major regulator of this specific set of alternative 3'-terminal exons. HuR binding to the alternative 3'-terminal exon in the pre-messenger RNA promotes its splicing, and is reduced by topoisomerase inhibitors. These findings provide new insights into the evolution, function and molecular regulation of alternative 3'-terminal exons.

RNA helicases DDX5 and DDX17 dynamically orchestrate transcription, miRNA, and splicing programs in cell differentiation.

The RNA helicases DDX5 and DDX17 are members of a large family of highly conserved proteins that are involved in gene-expression regulation; however, their in vivo targets and activities in biological processes such as cell differentiation, which requires reprogramming of gene-expression programs at multiple levels, are not well characterized. Here, we uncovered a mechanism by which DDX5 and DDX17 cooperate with heterogeneous nuclear ribonucleoprotein (hnRNP) H/F splicing factors to define epithelial- and myoblast-specific splicing subprograms. We then observed that downregulation of DDX5 and DDX17 protein expression during myogenesis and epithelial-to-mesenchymal transdifferentiation contributes to the switching of splicing programs during these processes. Remarkably, this downregulation is mediated by the production of miRNAs induced upon differentiation in a DDX5/DDX17-dependent manner. Since DDX5 and DDX17 also function as coregulators of master transcriptional regulators of differentiation, we propose to name these proteins "master orchestrators" of differentiation that dynamically orchestrate several layers of gene expression.

Splicing programs and cancer.

Numerous studies report splicing alterations in a multitude of cancers by using gene-by-gene analysis. However, understanding of the role of alternative splicing in cancer is now reaching a new level, thanks to the use of novel technologies allowing the analysis of splicing at a large-scale level. Genome-wide analyses of alternative splicing indicate that splicing alterations can affect the products of gene networks involved in key cellular programs. In addition, many splicing variants identified as being misregulated in cancer are expressed in normal tissues. These observations suggest that splicing programs contribute to specific cellular programs that are altered during cancer initiation and progression. Supporting this model, recent studies have identified splicing factors controlling cancer-associated splicing programs. The characterization of splicing programs and their regulation by splicing factors will allow a better understanding of the genetic mechanisms involved in cancer initiation and progression and the development of new therapeutic targets.

Splicing switch of an epigenetic regulator by RNA helicases promotes tumor-cell invasiveness.

Both epigenetic and splicing regulation contribute to tumor progression, but the potential links between these two levels of gene-expression regulation in pathogenesis are not well understood. Here, we report that the mouse and human RNA helicases Ddx17 and Ddx5 contribute to tumor-cell invasiveness by regulating alternative splicing of several DNA- and chromatin-binding factors, including the macroH2A1 histone. We show that macroH2A1 splicing isoforms differentially regulate the transcription of a set of genes involved in redox metabolism. In particular, the SOD3 gene that encodes the extracellular superoxide dismutase and plays a part in cell migration is regulated in an opposite manner by macroH2A1 splicing isoforms. These findings reveal a new regulatory pathway in which splicing factors control the expression of histone variant isoforms that in turn drive a transcription program to switch tumor cells to an invasive phenotype.

Estrogen regulation and physiopathologic significance of alternative promoters in breast cancer.

Alternative promoters (AP) occur in >30% protein-coding genes and contribute to proteome diversity. However, large-scale analyses of AP regulation are lacking, and little is known about their potential physiopathologic significance. To better understand the transcriptomic effect of estrogens, which play a major role in breast cancer, we analyzed gene and AP regulation by estradiol in MCF7 cells using pan-genomic exon arrays. We thereby identified novel estrogen-regulated genes (ERG) and determined the regulation of AP-encoded transcripts in 150 regulated genes. In <30% cases, APs were regulated in a similar manner by estradiol, whereas in >70% cases, they were regulated differentially. The patterns of AP regulation correlated with the patterns of estrogen receptor alpha (ERalpha) and CCCTC-binding factor (CTCF) binding sites at regulated gene loci. Interestingly, among genes with differentially regulated (DR) APs, we identified cases where estradiol regulated APs in an opposite manner, sometimes without affecting global gene expression levels. This promoter switch was mediated by the DDX5/DDX17 family of ERalpha coregulators. Finally, genes with DR promoters were preferentially involved in specific processes (e.g., cell structure and motility, and cell cycle). We show, in particular, that isoforms encoded by the NET1 gene APs, which are inversely regulated by estradiol, play distinct roles in cell adhesion and cell cycle regulation and that their expression is differentially associated with prognosis in ER(+) breast cancer. Altogether, this study identifies the patterns of AP regulation in ERGs and shows the contribution of AP-encoded isoforms to the estradiol-regulated transcriptome as well as their physiopathologic significance in breast cancer.

Exon-based clustering of murine breast tumor transcriptomes reveals alternative exons whose expression is associated with metastasis.

In the field of bioinformatics, exon profiling is a developing area of disease-associated transcriptome analysis. In this study, we performed a microarray-based transcriptome analysis at the single exon level in mouse 4T1 primary mammary tumors with different metastatic capabilities. A novel bioinformatics platform was developed that identified 679 genes with differentially expressed exons in 4T1 tumors, many of which were involved in cell morphology and movement. Of 152 alternative exons tested by reverse transcription-PCR, 97 were validated as differentially expressed in primary tumors with different metastatic capability. This analysis revealed candidate progression genes, hinting at variations in protein functions by alternate exon usage. In a parallel effort, we developed a novel exon-based clustering analysis and identified alternative exons in tumor transcriptomes that were associated with dissemination of primary tumor cells to sites of pulmonary metastasis. This analysis also revealed that the splicing events identified by comparing primary tumors were not aberrant events. Lastly, we found that a subset of differentially spliced variant transcripts identified in the murine model was associated with poor prognosis in a large clinical cohort of patients with breast cancer. Our findings illustrate the utility of exon profiling to define novel theranostic markers for study in cancer progression and metastasis.

Cotranscriptional exon skipping in the genotoxic stress response.

Pre-mRNA splicing is functionally coupled to transcription, and genotoxic stresses can enhance alternative exon inclusion by affecting elongating RNA polymerase II. We report here that various genotoxic stress inducers, including camptothecin (CPT), inhibit the interaction between Ewing's sarcoma proto-oncoprotein (EWS), an RNA polymerase II-associated factor, and YB-1, a spliceosome-associated factor. This results in the cotranscriptional skipping of several exons of the MDM2 gene, which encodes the main p53 ubiquitin ligase. This reversible exon skipping participates in the regulation of MDM2 expression that may contribute to the accumulation of p53 during stress exposure and its rapid shut-off when stress is removed. Finally, a splicing-sensitive microarray identified numerous exons that are skipped in response to CPT and EWS-YB-1 depletion. These data demonstrate genotoxic stress-induced alteration of the communication between the transcriptional and splicing machineries, which results in widespread exon skipping and plays a central role in the genotoxic stress response.

Widespread estrogen-dependent repression of micrornas involved in breast tumor cell growth.

Altered expression of microRNAs (miRNA), an abundant class of small nonprotein-coding RNAs that mostly function as negative regulators of protein-coding gene expression, is common in cancer. Here, we analyze the regulation of miRNA expression in response to estrogen, a steroid hormone that is involved in the development and progression of breast carcinomas and that is acting via the estrogen receptors (ER) transcription factors. We set out to thoroughly describe miRNA expression, by using miRNA microarrays and real-time reverse transcription-PCR (RT-PCR) experiments, in various breast tumor cell lines in which estrogen signaling has been induced by 17beta-estradiol (E(2)). We show that the expression of a broad set of miRNAs decreases following E(2) treatment in an ER-dependent manner. We further show that enforced expression of several of the repressed miRNAs reduces E(2)-dependent cell growth, thus linking expression of specific miRNAs with estrogen-dependent cellular response. In addition, a transcriptome analysis revealed that the E(2)-repressed miR-26a and miR-181a regulate many genes associated with cell growth and proliferation, including the progesterone receptor gene, a key actor in estrogen signaling. Strikingly, miRNA expression is also regulated in breast cancers of women who had received antiestrogen neoadjuvant therapy. Overall, our data indicate that the extensive alterations in miRNA regulation upon estrogen signaling pathway play a key role in estrogen-dependent functions and highlight the utility of considering miRNA expression in the understanding of antiestrogen resistance of breast cancer.

Cotranscriptional splicing potentiates the mRNA production from a subset of estradiol-stimulated genes.

While early steps of gene expression, such as transcription preinitiation, are known to often be rate limiting and to be regulated by such stimuli as steroid hormones, the potential impact of downstream steps, including splicing, on the mRNA production rate is unknown. In this work, we studied the effects of the transcriptional stimulus estradiol on cyclin D1, PS2, and c-fos gene expression by measuring the levels of RNA polymerase II on the DNA templates, the levels of nascent transcripts associated with RNA polymerase II, and the levels of unspliced, partially spliced, and fully spliced RNAs. We demonstrated that the efficiency of cotranscriptional splicing of the first intron was higher in the case of cyclin D1 than with PS2 and potentiated the cyclin D1 mRNA production rate. The mechanism involved in cotranscriptional splicing depended on the level of serine 5 phosphorylation of RNA polymerase II at the gene 5' end and on the recruitment of CBP80, one of the two subunits of the cap binding complex, which stimulates splicing of the promoter-proximal intron. Our data indicate that mRNA production from a subset of estradiol-stimulated genes, such as cyclin D1, could occur in a very efficient "assembly line." In contrast, we demonstrated for the first time that despite a strong transcriptional activation of the PS2 gene, the production of mRNA is not optimized owing to inefficient cotranscriptional RNA processing.