Abstract 1595: Phosphomimetic mutation of hnRNP A1 inhibits AKT-dependent IRES function and results in mTOR inhibitor sensitivity

Abstract

Abstract The relative activity of the Akt kinase has been demonstrated to be a major determinant of sensitivity of tumor cells to mTOR inhibitors. Our previous studies have shown that the multifunctional RNA-binding protein hnRNP A1 regulates a salvage pathway which facilitates IRES-dependent mRNA translation of critical cellular determinants in an Akt-dependent manner following mTOR inhibitor exposure. This salvage pathway functions by stimulating IRES-dependent translation in cells with relatively quiescent Akt resulting in resistance to rapamycin. However, the pathway is repressed in cells with elevated Akt activity rendering cells sensitive to rapamycin induced G1 arrest as a result of the inhibition of global eIF-4E-mediated translation. Akt phosphorylation of hnRNP A1 at serine 199 has been demonstrated to inhibit IRES-mediated translation initiation. Here we describe a phosphomimetic mutant of hnRNP A1 (S199E) which binds both the cyclin D1 and c-MYC IRES RNAs in vitro and results in inhibition of IRES function in dicistronic mRNA reporter assays. Utilizing cells in which Akt is conditionally active, we demonstrate that overexpression of this mutant renders quiescent Akt-containing cells sensitive to rapamycin in vitro and in xenografts. We also demonstrate that activated Akt is strongly correlated with elevated phospho- ser199-hnRNP A1 levels in a panel of 22 glioblastomas. These data demonstrate that the phosphorylation status of hnRNP A1 ser199 regulates the Akt-dependent sensitivity of cells to rapamycin and may have utility as a surrogate biomarker to stratify those tumors which may be most sensitive to mTOR inhibitor therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1595.