Heterogeneous Nuclear Ribonucleoprotein A1 Regulates Cyclin D1 and c-myc Internal Ribosome Entry Site Function through Akt Signaling

Department of Research and Development, Greater Los Angeles Veterans Affairs Healthcare System, Los Angeles, California 91343, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 07/2008; 283(34):23274-87. DOI: 10.1074/jbc.M801185200
Source: PubMed


The translation of the cyclin D1 and c-myc mRNAs occurs via internal ribosome entry site (IRES)-mediated initiation under conditions of reduced eIF-4F complex formation
and Akt activity. Here we identify hnRNP A1 as an IRES trans-acting factor that regulates cyclin D1 and c-myc IRES activity, depending on the Akt status of the cell. hnRNP A1 binds both IRESs in vitro and in intact cells and enhances in vitro IRES-dependent reporter expression. Akt regulates this IRES activity by inducing phosphorylation of hnRNP A1 on serine 199.
Serine 199-phosphorylated hnRNP A1 binds to the IRESs normally but is unable to support IRES activity in vitro. Reducing expression levels of hnRNP A1 or overexpressing a dominant negative version of the protein markedly inhibits rapamycin-stimulated
IRES activity in cells and correlated with redistribution of cyclin D1 and c-myc transcripts from heavy polysomes to monosomes. Importantly, knockdown of hnRNP A1 also renders quiescent Akt-containing cells
sensitive to rapamycin-induced G1 arrest. These results support a role for hnRNP A1 in mediating rapamycin-induced alterations of cyclin D1 and c-myc IRES activity in an Akt-dependent manner and provide the first direct link between Akt and the regulation of IRES activity.

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    • "On the other hand, phosphorylation of hnRNP A1 by AKT has no effects on splicing, but it modulates the translational activity of this RBP. Following phosphorylation, hnRNP A1 loses its ability to promote IRES dependent translation of the CCND1 and the c-MYC mRNAs [59]. In the case of SR proteins, AKT was shown to modulate both splicing and translational activity through phosphorylation. "
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    ABSTRACT: Alternative splicing (AS) is one of the key processes involved in the regulation of gene expression in eukaryotic cells. AS catalyzes the removal of intronic sequences and the joining of selected exons, thus ensuring the correct processing of the primary transcript into the mature mRNA. The combinatorial nature of AS allows a great expansion of the genome coding potential, as multiple splice-variants encoding for different proteins may arise from a single gene. Splicing is mediated by a large macromolecular complex, the spliceosome, whose activity needs a fine regulation exerted by cis-acting RNA sequence elements and trans-acting RNA binding proteins (RBP). The activity of both core spliceosomal components and accessory splicing factors is modulated by their reversible phosphorylation. The kinases and phosphatases involved in these posttranslational modifications significantly contribute to AS regulation and to its integration in the complex regulative network that controls gene expression in eukaryotic cells. Herein, we will review the major canonical and noncanonical splicing factor kinases and phosphatases, focusing on those whose activity has been implicated in the aberrant splicing events that characterize neoplastic transformation.
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    • "Recently, there has been intriguing evidence that AKT may regulate IRES-mediated translation at the level of ITAFs. In particular, AKT was shown to directly phosphorylate the ITAF hnHRP1A at serine 199 and inhibit IRES-mediated translation (Jo et al, 2008). In this way, AKT may actively limit IRES-mediated translation through phosphorylation of ITAFs while it simultaneously promotes cap-dependent translation (see above). "
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    ABSTRACT: The AKT signalling pathway is a major regulator of protein synthesis that impinges on multiple cellular processes frequently altered in cancer, such as proliferation, cell growth, survival, and angiogenesis. AKT controls protein synthesis by regulating the multistep process of mRNA translation at every stage from ribosome biogenesis to translation initiation and elongation. Recent studies have highlighted the ability of oncogenic AKT to drive cellular transformation by altering gene expression at the translational level. Oncogenic AKT signalling leads to both global changes in protein synthesis as well as specific changes in the translation of select mRNAs. New and developing technologies are significantly advancing our ability to identify and functionally group these translationally controlled mRNAs into gene networks based on their modes of regulation. How oncogenic AKT activates ribosome biogenesis, translation initiation, and translational elongation to regulate these translational networks is an ongoing area of research. Currently, the majority of therapeutics targeting translational control are focused on blocking translation initiation through inhibition of eIF4E hyperactivity. However, it will be important to determine whether combined inhibition of ribosome biogenesis, translation initiation, and translation elongation can demonstrate improved therapeutic efficacy in tumours driven by oncogenic AKT.
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    • "As shown in Fig. 1E and Supplemental Fig. 1, both types of embryos exhibited comparable amounts and similar distribution patterns of Runx1 protein, indicating that IRES deficiency did not significantly affect the Runx1 protein expression. This finding led us to the idea that IRES-mediated Runx1 expression might be essential in a specific developmental stage, a particular cell lineage and/or a critical point of cell cycle, as suggested in previous studies (Mitchell et al., 2003; Gonzalez-Herrera et al., 2006; Audigier et al., 2008; Dobbyn et al., 2008; Jo et al., 2008). "
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    ABSTRACT: Although internal ribosome entry site (IRES)-mediated translation is considered important for proper cellular function, its precise biological role is not fully understood. Runx1 gene, which encodes a transcription factor implicated in hematopoiesis, angiogenesis, and leukemogenesis, contains IRES sequences in the 5' untranslated region. To clarify the roles of the IRES element in Runx1 function, we generated knock-in mice for either wild-type Runx1 or Runx1/Evi1, a Runx1 fusion protein identified in human leukemia. In both cases, native promoter-dependent transcription was retained, whereas IRES-mediated translation was eliminated. Interestingly, homozygotes expressing wild-type Runx1 deleted for the IRES element (Runx1(Delta IRES/Delta IRES)) died in utero with prominent dilatation of peripheral blood vessels due to impaired pericyte development. In addition, hematopoietic cells in the Runx1(Delta IRES/Delta IRES) fetal liver were significantly decreased, and exhibited an altered differentiation pattern, a reduced proliferative activity, and an impaired reconstitution ability. On the other hand, heterozygotes expressing Runx1/Evi1 deleted for the IRES element (Runx1(+/RE Delta IRES)) were born normally and did not show any hematological abnormalities, in contrast that conventional Runx1/Evi1 heterozygotes die in utero with central nervous system hemorrhage and Runx1/Evi1 chimeric mice develop acute leukemia. The findings reported here demonstrate the essential roles of the IRES element in Runx1 function under physiological and pathological conditions.
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