title: Phosphatidylinositol-3 kinase-dependent translational regulation of Id1 involves the PPM1G phosphatase.
abstract: Id1 is a helix-loop-helix transcriptional modulator that increases the aggressiveness of malignant glial neoplasms. Since most glioblastomas (GBMs) show increased phosphatidylinositol-3 kinase (PI-3K) signaling, we sought to determine whether this pathway regulates Id1 expression. Higher basal Id1 expression correlates with dysregulated PI-3K signaling in multiple established GBM cell lines. Further characterization of PI-3K-dependent Id1 regulation reveals that chemical or genetic inhibition of PI-3K signaling reduces Id1 protein but not mRNA expression. Overall, PI-3K signaling appears to enhance Id1 translation with no significant effect on its stability. PI-3K signaling is known to regulate protein translation through mTORC1-dependent phosphorylation of 4E-BP1, which reduces its association with and inhibition of the translation initiation factor eIF4E. Interestingly, while inhibition of PI-3K and AKT lowers 4E-BP1 phosphorylation and expression of Id1 in all cases, inhibition of TORC1 with rapamycin does not consistently have a similar effect, suggesting an alternative mechanism for PI-3K-dependent regulation of Id1 translation. We now identify a potential role for the serine-threonine phosphatase PPM1G in translational regulation of Id1 protein expression. PPM1G knockdown by siRNA increase both 4E-BP1 phosphorylation and Id1 expression and PPM1G and 4E-BP1 co-associates in GBM cells. Furthermore, PPM1G is a phosphoprotein and this phosphorylation appears to be regulated by PI-3K activity. Finally, PI-3K inhibition increases PPM1G activity when assessed by an in vitro phosphatase assay. Our findings provide the first evidence that the PI-3K/AKT signaling pathway modulates PPM1G activity resulting in a shift in the balance between hyper- and hypo-phosphorylated 4E-BP1 and translational regulation of Id1 expression.
authors.affiliation: Department of Radiation Oncology and the Winship Cancer Institute, Emory University, Atlanta, GA, USA.
authors.forename: K
authors.initials: K
authors.lastname: Xu
authors.affiliation: Department of Radiation Oncology and the Winship Cancer Institute, Emory University, Atlanta, GA, USA.
authors.forename: L
authors.initials: L
authors.lastname: Wang
authors.affiliation: Department of Pharmacology, Emory University, Atlanta, GA, USA.
authors.forename: W
authors.initials: W
authors.lastname: Feng
authors.affiliation: Department of Pharmacology, Emory University, Atlanta, GA, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Feng
authors.affiliation: Department of Radiation Oncology and the Winship Cancer Institute, Emory University, Atlanta, GA, USA.
authors.forename: H-Kg
authors.initials: HK
authors.lastname: Shu
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D018797:Cell Cycle Proteins; D045744:Cell Line, Tumor; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D051798:Inhibitor of Differentiation Protein 1 / Q000235:genetics*; D000076222:Mechanistic Target of Rapamycin Complex 1; D008954:Models, Biological; D046912:Multiprotein Complexes / Q000378:metabolism; D058539:Phosphatidylinositol 3-Kinase / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D014176:Protein Biosynthesis; D000071636:Protein Phosphatase 2C / Q000235:genetics / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D018797:Cell Cycle Proteins; C087000:EIF4EBP1 protein, human; D051798:Inhibitor of Differentiation Protein 1; D046912:Multiprotein Complexes; D010750:Phosphoproteins; D058539:Phosphatidylinositol 3-Kinase; D000076222:Mechanistic Target of Rapamycin Complex 1; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; D000071636:Protein Phosphatase 2C; D020123:Sirolimus