ALK-mediated post-transcriptional regulation: focus on RNA-binding proteins.

Extensive research has been carried out in the past two decades to provide insights into the molecular mechanisms by which the Nucleophosmin-Anaplastic Lymphoma Kinase (NPM-ALK) exerts its oncogenic effects. These studies led to the concept that NPM-ALK acts at the transcriptional level through the activation of several transcription factors downstream of many different signaling pathways including JAK3/STAT3, PI3K/AKT and RAS/ERK. Nevertheless, the discovery of several RNA-binding proteins (RBPs) within ALK interactome suggested an additional and complementary role of this oncogenic kinase at the post-transcriptional level.

RNA-binding proteins, RNA granules, and gametes: is unity strength?

Changes in mRNA translation and degradation represent post-transcriptional processes operating during gametogenesis and early embryogenesis to ensure regulated protein synthesis. Numerous mRNA-binding proteins (RBPs) have been described in multiple animal models that contribute to the control of mRNA translation and decay during oogenesis and spermatogenesis. An emerging view from studies performed in germ cells and somatic cells is that RBPs associate with their target mRNAs in RNA-protein (or ribonucleoprotein) complexes (mRNPs) that assemble in various cytoplasmic RNA granules that communicate with the translation machinery and control mRNA storage, triage, and degradation.

Looking for the functions of RNA granules in ALK-transformed cells.

Numerous cytoplasmic foci containing mRNA s and their associated proteins have been described in mammalian somatic and germ cells. The best studied examples are given by the processing bodies (PBs) that are present in all cell types, and the stress granules (SGs) that are transiently formed following stress stimuli. Those foci are non-membranous dynamic structures that, through the continuous exchange of their content with the cytoplasm, are believed to control mRNA storage, translation and degradation.

The RNA-binding protein ELAVL1/HuR is essential for mouse spermatogenesis, acting both at meiotic and postmeiotic stages.

Posttranscriptional mechanisms are crucial to regulate spermatogenesis. Accurate protein synthesis during germ cell development relies on RNA binding proteins that control the storage, stability, and translation of mRNAs in a tightly and temporally regulated manner. Here, we focused on the RNA binding protein Embryonic Lethal Abnormal Vision (ELAV) L1/Human antigen R (HuR) known to be a key regulator of posttranscriptional regulation in somatic cells but the function of which during gametogenesis has never been investigated.

Stabilization of Dll1 mRNA by Elavl1/HuR in neuroepithelial cells undergoing mitosis.

In the vertebrate neuroepithelium, the decision to differentiate is made by neural precursors soon after mitosis, when they are apically located. This process is controlled by lateral inhibitory signals triggered by the Delta/Notch pathway. During mitosis, the capacity of neural precursors to express the neurogenic genes Dll1 and Notch1 is maximal due to mRNA stabilization, but the mechanism controlling this process remains unknown.

HuR-mediated control of C/EBPbeta mRNA stability and translation in ALK-positive anaplastic large cell lymphomas.

The CCAAT/enhancer-binding protein β (C/EBPβ) plays a major role in the pathogenesis of anaplastic large cell lymphomas (ALCL) that express the nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) tyrosine kinase (ALK(+)). Although ALK-mediated C/EBPβ transcriptional activation has been reported, C/EBPβ mRNA possesses U- and AU-rich domains in its 3'-untranslated region (3'-UTR) that might be privileged targets for posttranscriptional control in ALK(+) ALCLs. The purpose of this study was to explore this possibility.

Novel mRNA-containing cytoplasmic granules in ALK-transformed cells.

In mammalian cells, nontranslating messenger RNAs (mRNAs) are concentrated in different cytoplasmic foci, such as processing bodies (PBs) and stress granules (SGs), where they are either degraded or stored. In the present study, we have thoroughly characterized cytoplasmic foci, hereafter called AGs for ALK granules that form in transformed cells expressing the constitutively active anaplastic lymphoma kinase (ALK). AGs contain polyadenylated mRNAs and a unique combination of several RNA binding proteins that so far has not been described in mammalian foci, including AUF1, HuR, and the poly (A(+)) binding protein PABP.

Impaired embryonic development in mice overexpressing the RNA-binding protein TIAR.

Background: TIA-1-related (TIAR) protein is a shuttling RNA-binding protein involved in several steps of RNA metabolism. While in the nucleus TIAR participates to alternative splicing events, in the cytoplasm TIAR acts as a translational repressor on specific transcripts such as those containing AU-Rich Elements (AREs). Due to its ability to assemble abortive pre-initiation complexes coalescing into cytoplasmic granules called stress granules, TIAR is also involved in the general translational arrest observed in cells exposed to environmental stress.

zeta-Crystallin is a bcl-2 mRNA binding protein involved in bcl-2 overexpression in T-cell acute lymphocytic leukemia.

The human antiapoptotic bcl-2 gene has been discovered in t(14;18) B-cell leukemias/lymphomas because of its overexpression caused at a transcriptional control level by the bcl-2/IgH fusion gene. We were the first to disclose the post-transcriptional control of bcl-2 expression mediated by interactions of an adenine + uracil (AU)-rich element (ARE) in the 3'-UTR of bcl-2 mRNA with AU-binding proteins (AUBPs). Here, we identify and characterize zeta-crystallin as a new bcl-2 AUBP, whose silencing or overexpression has impact on bcl-2 mRNA stability.

Temporally regulated traffic of HuR and its associated ARE-containing mRNAs from the chromatoid body to polysomes during mouse spermatogenesis.

Background: In mammals, a temporal disconnection between mRNA transcription and protein synthesis occurs during late steps of germ cell differentiation, in contrast to most somatic tissues where transcription and translation are closely linked. Indeed, during late stages of spermatogenesis, protein synthesis relies on the appropriate storage of translationally inactive mRNAs in transcriptionally silent spermatids. The factors and cellular compartments regulating mRNA storage and the timing of their translation are still poorly understood.

AU-rich elements regulate Drosophila gene expression.

In mammals, AU-rich elements (AREs) are critical regulators of mRNA turnover. They recruit ARE-binding proteins that inhibit or stimulate rapid mRNA degradation in response to stress or developmental cues. Using a bioinformatics approach, we have identified AREs in Drosophila melanogaster 3' untranslated regions and validated their cross-species conservation in distant Drosophila genomes.

[Aberrant regulation of mRNA 3' untranslated region in cancers and inflammation].

Almost 10% of mammalian coding mRNAs contain in their 3' untranslated region a sequence rich in adenine and uridine residues known as AU-rich element (ARE). Many of them encode oncogenes (for instance c-Myc and c-Fos), cell cycle regulators (cyclin D1, A1, B1), cytokines (TNFalpha, IL2) and growth factors (GM-CSF) which are overexpressed in cancer or inflammatory diseases due to increased mRNA stability and/or translation. AREs are recognized by a group of proteins, collectively called AUBPs which display various functions.

Low dietary inorganic phosphate affects the brain by controlling apoptosis, cell cycle and protein translation.

Inorganic phosphate (Pi) plays a key role in diverse physiologic functions. In a previous study, we showed that high dietary Pi perturbs brain growth through Akt/ERK signaling in developing mice. However, no study has investigated the response of the brain to low dietary Pi.

A "liaison dangereuse" between AUF1/hnRNPD and the oncogenic tyrosine kinase NPM-ALK.

Nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) is a chimeric protein expressed in a subset of cases of anaplastic large cell lymphoma (ALCL) for which constitutive expression represents a key oncogenic event. The ALK signaling pathway is complex and probably involves functional redundancy between various signaling substrates of ALK. Despite numerous studies on signaling mediators, the molecular mechanisms contributing to the distinct oncogenic features of NPM-ALK remain incompletely understood.

A high inorganic phosphate diet perturbs brain growth, alters Akt-ERK signaling, and results in changes in cap-dependent translation.

Inorganic phosphate (Pi) plays a key role in diverse physiological functions. Recently, considerable progress has been made in our understanding of the function and regulation of the brain-specific sodium-dependent inorganic phosphate transporter 1 (NPT1), which is found to exist principally in cerebrum and cerebellum. The potential importance of Pi as a novel signaling molecule and the poor prognosis of diverse neurodegenerative diseases that involve brain-specific NPT1 have prompted us to define the pathways by which Pi affects mouse brain growth.

Expression patterns of the coe/ebf transcription factor genes during chicken and mouse limb development.

The COE (Collier/Olf/EBF) family of transcription factors comprises a single member in Drosophila and four members in human and mice. We have examined by in situ hybridization the expression patterns of each ebf/coe gene during limb development in mouse and chicken embryos. Expression of mouse ebf1, 2 and 3 is detected in mesenchymal cells from stages E10.

Evidence of finely tuned expression of DNA polymerase beta in vivo using transgenic mice.

DNA polymerase (Pol) is an error-prone repair DNA polymerase that has been shown to create genetic instability and tumorigenesis when overexpressed by only 2-fold in cells, suggesting that a rigorous regulation of its expression may be essential in vivo. To address this question, we have generated mice which express a transgene (Tg) bearing the Pol cDNA under the control of the ubiquitous promoter of the mouse H-2K gene from the major histocompatibility complex. These mice express the Tg only in thymus, an organ which normally contains the most abundant endogenous Pol mRNA and protein, supporting the idea of a tight regulation of Pol in vivo.

Modulo is a target of Myc selectively required for growth of proliferative cells in Drosophila.

In Drosophila, the homologue of the proto-oncogene Myc is a key regulator of both cell size and cell growth. The identities and roles of dMyc target genes in these processes, however, remain largely unexplored. Here, we investigate the function of the modulo (mod) gene, which encodes a nucleolus localized protein.

Impaired gametogenesis in mice that overexpress the RNA-binding protein HuR.

A series of experiments, using cell culture models or in vitro assays, has shown that the RNA-binding protein HuR increases the half-life of some messenger RNAs that contain adenylate/uridylate-rich decay elements. However, its function in an integrated system has not yet been investigated. Here, using a mouse model, we report that misregulation of HuR, due to expression of an HuR transgene, prevents the production of fully functional gametes.

A new player in oncogenesis: AUF1/hnRNPD overexpression leads to tumorigenesis in transgenic mice.

AUF1/heterogeneous nuclear ribonucleoprotein D (hnRNPD) binds to adenylate uridylate-rich elements contained in the 3' untranslated region of many short-lived mRNAs. This binding has been shown in vitro to control the stability of adenylate uridylate-rich element-containing mRNAs, including mRNAs encoding proto-oncogenes, cytokines, or other signaling molecules. However, no studies have yet been undertaken to identify the mRNAs subject to AUF1-mediated regulation in vivo.