Akt phosphorylation and nuclear phosphoinositide association mediate mRNA export and cell proliferation activities by ALY. Proceedings of the National Academy of Sciences of the United States of America

Department of Pathology and Laboratory Medicine, Emory University School of Medicine, Atlanta, GA 30322, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 07/2008; 105(25):8649-54. DOI: 10.1073/pnas.0802533105
Source: PubMed


Nuclear PI3K and its downstream effectors play essential roles in a variety of cellular activities including cell proliferation, survival, differentiation, and pre-mRNA splicing. Aly is a nuclear speckle protein implicated in mRNA export. Here we show that Aly is a physiological target of nuclear PI3K signaling, which regulates its subnuclear residency, cell proliferation, and mRNA export activities through nuclear Akt phosphorylation and phosphoinositide association. Nuclear Akt phosphorylates Aly on threonine-219, which is required for its interaction with Akt. Aly binds phosphoinositides, and this action is regulated by Akt-mediated phosphorylation. Phosphoinositide binding but not Akt phosphorylation dictates Aly's nuclear speckle residency. Depletion of Aly results in cell growth suppression and mRNA export reduction. Inhibition of Aly phosphorylation substantially decreases cell proliferation and mRNA export. Furthermore, disruption of phosphoinositide association with Aly also significantly reduces these activities. Thus, nuclear PI3K signaling mediates both cell proliferation and mRNA export functions of Aly.

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    • "IPMK deletion causes a marked reduction in cellular PIP 3 levels, accompanied by impaired activation of the enzyme AKT (Maag et al., 2011). Because AKT activity has been implicated in the binding of ALY to phosphoinositides (Okada et al., 2008), and in the regulation of nuclear mRNA export via TREX components (Okada et al., 2008), we tested the effect of chemical inhibition of AKT on ALY recognition of the 3 0 UTR of RAD51. Interestingly, we find that AKT inhibition causes a decrease in ALY recognition, and that this can be partially restored by the addition of PIP 3 despite the continuing presence of the AKT inhibitor (Figure S6D). "
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    ABSTRACT: Messenger RNA (mRNA) export from the nucleus is essential for eukaryotic gene expression. Here we identify a transcript-selective nuclear export mechanism affecting certain human transcripts, enriched for functions in genome duplication and repair, controlled by inositol polyphosphate multikinase (IPMK), an enzyme catalyzing inositol polyphosphate and phosphoinositide turnover. We studied transcripts encoding RAD51, a protein essential for DNA repair by homologous recombination (HR), to characterize the mechanism underlying IPMK-regulated mRNA export. IPMK depletion or catalytic inactivation selectively decreases RAD51 protein abundance and the nuclear export of RAD51 mRNA, thereby impairing HR. Recognition of a sequence motif in the untranslated region of RAD51 transcripts by the mRNA export factor ALY requires IPMK. Phosphatidylinositol (3,4,5)-trisphosphate (PIP3), an IPMK product, restores ALY recognition in IPMK-depleted cell extracts, suggesting a mechanism underlying transcript selection. Our findings implicate IPMK in a transcript-selective mRNA export pathway controlled by phosphoinositide turnover that preserves genome integrity in humans.
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    • "Interestingly, treatment with ET-18-OCH3 prior to gp120 exposure did not significantly affect CCL2 transcript accumulation, thus adding further evidence that PI-PLC is not a component of the gp120-stimulated, NF-kB-mediated pathway leading to CCL2 transcript accumulation. Since it has been previously shown that nuclear PIs play a critical role in mRNA processing and export [49], [50], and that PI-PLC β1 is the key enzyme involved in the cycle of nuclear PIs [51], we foresaw to determine whether ET-18-OCH3 could affect CCL2 mRNA nuclear export in MDM. To this aim, cytoplasmic RNA was extracted from cells treated or not with ET-18-OCH3 prior to gp120 exposure. "
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    • "One branch of the phosphatidylinositol signal transduction pathway, downstream from PI3 kinase, operates through AKT (Barnett et al., 2005; Franke, 2008). AKT has been reported to phosphorylate the mRNA export linker protein ALY/REF in vitro (Okada et al., 2008). Point mutants that cannot be AKT FIGURE 7: The AKT signaling pathway does not affect the nuclear export of mRNAs encoding either ER-targeted or nuclear-encoded mitochondrial proteins. "
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    ABSTRACT: UAP56, ALY/REF, and NXF1 are mRNA export factors that sequentially bind at the 5' end of a nuclear mRNA, but are also reported to associate with the Exon Junction Complex (EJC). To screen for signal transduction pathways regulating mRNA export complex assembly we used Fluorescence Recovery after Photobleaching (FRAP) to measure the binding of mRNA export and EJC core proteins in nuclear complexes. The fraction of UAP56, ALY/REF, and NXF1 tightly bound in complexes was reduced by drug inhibition of the PI3 kinase / AKT pathway, as was the tightly bound fraction of the core EJC proteins eIF4A3, MAGOH, and Y14. Inhibition of the mTOR mTORC1 pathway decreased the tight binding of MAGOH. Inhibition of the PI3 Kinase/AKT pathway increased the export of poly(A) RNA and of a subset of candidate mRNAs. A similar effect of PI3 kinase/AKT inhibition was observed for mRNAs from both intron-containing and intron-less Histone genes. However, the nuclear export of mRNAs coding for proteins targeted to the Endoplasmic Reticulum or to Mitochondria was not affected by the PI3 kinase/AKT pathway. These results show that the active PI3 kinase/AKT pathway can regulate mRNA export and can promote the nuclear retention of some mRNAs.
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