hnRNP H/F drive RNA G-quadruplex-mediated translation linked to genomic instability and therapy resistance in glioblastoma.

RNA G-quadruplexes (RG4s) are four-stranded structures known to control mRNA translation of cancer relevant genes. RG4 formation is pervasive in vitro but not in cellulo, indicating the existence of poorly characterized molecular machinery that remodels RG4s and maintains them unfolded. Here, we performed a quantitative proteomic screen to identify cytosolic proteins that interact with a canonical RG4 in its folded and unfolded conformation. Our results identified hnRNP H/F as important components of the cytoplasmic machinery modulating the structural integrity of RG4s, revealed their function in RG4-mediated translation and uncovered the underlying molecular mechanism impacting the cellular stress response linked to the outcome of glioblastoma.

Key contribution of eIF4H-mediated translational control in tumor promotion.

Dysregulated expression of translation initiation factors has been associated with carcinogenesis, but underlying mechanisms remains to be fully understood. Here we show that eIF4H (eukaryotic translation initiation factor 4H), an activator of the RNA helicase eIF4A, is overexpressed in lung carcinomas and predictive of response to chemotherapy. In lung cancer cells, depletion of eIF4H enhances sensitization to chemotherapy, decreases cell migration and inhibits tumor growth in vivo, in association with reduced translation of mRNA encoding cell-proliferation (c-Myc, cyclin D1) angiogenic (FGF-2) and anti-apoptotic factors (CIAP-1, BCL-xL). Conversely, each isoform of eIF4H acts as an oncogene in NIH3T3 cells by stimulating transformation, invasion, tumor growth and resistance to drug-induced apoptosis together with increased translation of IRES-containing or structured 5'UTR mRNAs. These results demonstrate that eIF4H plays a crucial role in translational control and can promote cellular transformation by preferentially regulating the translation of potent growth and survival factor mRNAs, indicating that eIF4H is a promising new molecular target for cancer therapy.

A synonymous polymorphism of the Tristetraprolin (TTP) gene, an AU-rich mRNA-binding protein, affects translation efficiency and response to Herceptin treatment in breast cancer patients.

Post-transcriptional regulation plays a central role in cell differentiation and proliferation. Among the regulatory factors involved in this mechanism, Tristetraprolin (ZFP36 or TTP) is the prototype of a family of RNA-binding proteins that bind to adenylate and uridylate (AU)-rich sequences in the 3'UTR of mRNAs, which promotes their physiological decay. Here, we investigated whether TTP correlates with tumor aggressiveness in breast cancer and is a novel prognostic factor for this neoplasia. By immunoblot analysis, we determined the amount of TTP protein in different breast cancer cell lines and found an inverse correlation between aggressiveness and metastatic potential. TTP mRNA levels were very variable among cells lines and did not correlate with protein levels. Interestingly, by sequencing the entire TTP coding region in Hs578T cells that do not express the TTP protein, we identified a synonymous polymorphism (rs3746083) that showed a statistically significant association with a lack of response to Herceptin/Trastuzumab in HER2-positive-breast cancer patients. Even though this genetic change did not modify the corresponding amino acid, we performed functional studies and showed an effect on protein translation associated with the variant allele with respect to the wild-type. These data underline the importance of synonymous variants on gene expression and the potential role of TTP genetic polymorphisms as a prognostic marker for breast cancer.

Alternative-splicing-based bicistronic vectors for ratio-controlled protein expression and application to recombinant antibody production.

In the last decade polycistronic vectors have become essential tools for both basic science and gene therapy applications. In order to co-express heterologous polypeptides, different systems have been developed from Internal Ribosome Entry Site (IRES) based vectors to the use of the 2A peptide. Unfortunately, these methods are not fully suitable for the efficient and reproducible modulation of the ratio between the proteins of interest. Here we describe a novel bicistronic vector type based on the use of alternative splicing. By modifying the consensus sequence that governs splicing, we demonstrate that the ratio between the synthesized proteins could easily vary from 1 : 10 to 10 : 1. We have established this system with luciferase genes and we extended its application to the production of recombinant monoclonal antibodies. We have shown that these vectors could be used in several typical cell lines with similar efficiencies. We also present an adaptation of these vectors to hybrid alternative splicing/IRES constructs that allow a ratio-controlled expression of proteins of interest in stably transfected cell lines.

An upstream open reading frame within an IRES controls expression of a specific VEGF-A isoform.

Vascular endothelial growth factor A (VEGF-A) is a potent secreted mitogen critical for physiological and pathological angiogenesis. Regulation of VEGF-A occurs at multiple levels, including transcription, mRNA stabilization, splicing, translation and differential cellular localization of various isoforms. Recent advances in our understanding of the posttranscriptional regulation of VEGF-A are comprised of the identification of stabilizing mRNA-binding proteins and the discovery of two internal ribosomal entry sites (IRES) as well as two alternative initiation codons in the 5'UTR of the VEGF-A mRNA. We have previously reported that VEGF-A translation initiation at both the AUG and CUG codons is dependent on the exon content of the coding region. In this report, we show that the expression of different VEGF-A isoforms is regulated by a small upstream open reading frame (uORF) located within an internal ribosome entry site, which is translated through a cap-independent mechanism. This uORF acts as a cis-regulatory element that regulates negatively the expression of the VEGF 121 isoform. Our data provide a framework for understanding how VEGF-A mRNAs are translated, and how the production of the VEGF 121 isoform is secured under non-hypoxic environmental conditions.

TPM3-ALK expression induces changes in cytoskeleton organisation and confers higher metastatic capacities than other ALK fusion proteins.

Translocations of the anaplastic lymphoma kinase (ALK) gene result in the production of a number of oncogenic ALK fusion proteins implicated in tumour development. We have previously shown that X-ALK fusion proteins have differential effects on the proliferation, transformation, and invasion properties of NIH3T3 cells in vitro. In the present study, we have investigated the metastatic potential of various X-ALK expressing cell lines using an experimental lung metastasis assay. We have shown that TPM3-ALK expression bestows higher metastatic capacities than other X-ALK fusion proteins and in addition, that TPM3-ALK fusion protein expression specifically induces changes in cell morphology and cytoskeleton organisation. Co-immunoprecipitation studies demonstrate a specific interaction between TPM3-ALK and endogenous tropomyosin. Together the specific actions of TPM3-ALK on the cytoskeleton organisation offer an interesting hypothesis with respect to the higher cell motility and metastatic potential of this fusion protein.

Differential effects of X-ALK fusion proteins on proliferation, transformation, and invasion properties of NIH3T3 cells.

Majority of anaplastic large-cell lymphomas (ALCLs) are associated with the t(2;5)(p23;q35) translocation, fusing the NPM (nucleophosmin) and ALK (anaplastic lymphoma kinase) genes (NPM-ALK). Recent studies demonstrated that ALK may also be involved in variant translocations, namely, t(1;2)(q25;p23), t(2;3)(p23;q21), t(2;17)(p23;q23) and inv(2)(p23q35), which create the TPM3-ALK, TFG-ALK5, CLTC-ALK, and ATIC-ALK fusion genes, respectively. Although overexpression of NPM-ALK has previously been shown to transform fibroblasts, the transforming potential of variant X-ALK proteins has not been precisely investigated. We stably transfected the cDNAs coding for NPM-ALK, TPM3-ALK, TFG-ALK, CLTC-ALK or ATIC-ALK into nonmalignant NIH3T3 cells. All X-ALK variants are tyrosine phosphorylated and their subcellular distribution was in agreement with that observed in tumors. Moreover, our results show that the in vitro transforming capacity of NIH3T3-transfected cells are in relation to the level of X-ALK fusion proteins excepted for TPM3-ALK for which there is an inverse correlation. The differences between the five X-ALK variants with regard to proliferation rate, colony formation in soft agar, invasion, migration through the endothelial barrier and tumorigenicity seem to be due to differential activation of various signaling pathways such as PI3-kinase/AKT. These findings may have clinical implications in the pathogenesis and prognosis of ALK-positive ALCLs.

Control of the vascular endothelial growth factor internal ribosome entry site (IRES) activity and translation initiation by alternatively spliced coding sequences.

The vascular endothelial growth factor-A (VEGF) gene locus contains eight exons that span 14 kb. Alternative splicing generates multiple, different mRNAs that in turn translate into at least five protein isoforms. While the canonical AUG start codon is located at position 1039 in exon 1, there also exists an upstream, in-frame CUG initiation codon that drives expression of L-VEGF, containing an additional 180 amino acids. Two separate internal ribosome entry sites (IRES) regulate the activity of each initiation codon. Thus the 5'-UTR of VEGF, which comprises the majority of exon 1, consists of IRES B, the CUG, IRES A, and the AUG, from 5' to 3'. Previously, it has been shown that IRES B regulates initiation at the CUG and IRES A regulates AUG usage. In this study, we have found evidence that the exon content of the VEGF mRNA, determined through alternative splicing, controls IRES A activity. While the CUG is most efficient at initiating translation, transcripts that lack both exons 6 and 7 and therefore contain an exon 5/8 junction lack AUG-initiated translation. The process of splicing is not responsible for this start codon selection since transfection of genomic and cDNA VEGF sequences give the same expression pattern. We hypothesize that long range tertiary interactions in the VEGF mRNA regulate IRES activity and thus control start codon selection. This is the first report describing the influence of alternatively spliced coding sequences on codon selection by modulating IRES activity.