Akt phosphorylation and nuclear phosphoinositide association mediate mRNA export and cell proliferation activities by ALY

Abstract

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.nuclear PI 3-kinase ͉ nuclear speckle ͉ nuclear Akt ͉ T219 phosphorylation P I3K and Akt predominantly locate in the cytoplasm, but they also occur in the nucleus or translocate there upon stimulation. Most of phosphoinositol lipid metabolic enzymes and PI3K signaling machinery occur in the nucleus. For example, PI-PLC (1, 2), phosphatidylinositol phosphate kinases (3, 4), PDK1 (5), PTEN (6 -8), and Akt (9, 10). Nuclear phosphoinositides play critical roles in cell growth and differentiation (11). In addition to nuclear membrane, phosphoinositides also associate with the nuclear speckles, a peculiar nuclear subcompartment enriched in small ribonucleoprotein (RNP) particles and various splicing factors (12), where many elements of nuclear phosphoinositide metabolism, including PI(4,5)P 2 , are concentrated (4,13,14). Nuclear phosphoinositides may be implicated in pre-mRNA splicing and chromatin structure (11). It has been shown before that nuclear PI(4,5)P 2 assembles in a mitotically regulated particle involved in pre-mRNA splicing (14). Elements of the transcriptional and pre-mRNA processing machinery interact with this pool of nuclear PI(4,5)P 2 , and PI(4,5)P 2 immunoprecipitates contain intermediates and products of the splicing reaction. Alternative splicing of pre-mRNAs can be regulated by extracellular signals such as growth factors, cytokines, hormones, and stress stimuli (15). For instance, insulin-activated PI3K signaling has been shown to implicate in mammary epithelial-mesenchymal interaction, which regulates fibronectin alternative splicing (16).Splicing of pre-mRNA and export of mRNA are normally coupled (17-19). Pre-mRNA splicing has been proposed to stimulate mRNA export (17,20). Based on genetic and biochemical evidence, Reed et al. (21) proposed a model for mRNA export: nascent pre-mRNA is first packaged into heterogeneous nuclear RNP (hnRNP) particles. During spliceosome assembly,...

Involvement of PIP2 in RNA Polymerase I transcription

“…Moreover, apart from mRNA processing, mRNA export was also shown to be regulated by PIP2 in speckles. PIP2 binding to the mRNA export protein Aly directed the protein to nuclear speckles, while the disruption of PIP2 binding caused a reduction in its mRNA export activity (Okada et al, 2008). A few studies showed the involvement of PIP2 in RNA polymerase II (Pol II) transcription.…”

Viral Factors Reveal a Role for REF/Aly in Nuclear RNA Stability

“…Small interfering RNA (siRNA)-mediated knockdown of REF family members decreases bulk mRNA export to various degrees in metazoan cells. In some cases, little or no nuclear poly(A) accumulation is observed, while other studies report an accumulation of poly(A) RNA in the nucleus (15,19,25,37,43,55). These distinct phenotypes may be due to redundancy with other adaptors (78), compensatory changes in gene expression (25), and/or differences in experimental procedures.…”

Phosphatidic Acid Interacts with a MYB Transcription Factor and Regulates Its Nuclear Localization and Function inArabidopsis

“…On the other hand, in Arabidopsis, PtdIns(5)P promotes translocation of antioxidant protein1, a histone 3 Lys4 trimethyltransferase, from the nucleus to the cytoplasm (Alvarez-Venegas et al, 2006). PtdIns(4,5)P 2 is a regulator of protein-protein and protein-nucleic acid interactions (Yu et al, 1998;Zhao et al, 1998;Krylova et al, 2005;Mellman et al, 2008;Okada et al, 2008;Barlow et al, 2010). In addition, PtdIns(4,5)P 2 is also an activator of PLDz that produces PA (Testerink et al, 2007).…”

“…5)P 2 ] binds effector proteins to affect their nuclear localization, gene expression, RNA processing, mRNA export, and chromatin unfolding in animal cells (Yu et al, 1998;Zhao et al, 1998;Krylova et al, 2005;Mellman et al, 2008;Okada et al, 2008;Barlow et al, 2010). Phosphatidylinositol-5-bisphosphate [PtdIns(5)P] has been reported to promote protein translocation to the nucleus (Gozani et al, 2003) and also from the nucleus to the cytoplasm (Alvarez-Venegas et al, 2006;Viiri et al, 2009;Dieck et al, 2012).…”