Abstract 4195: Protein phosphatase PPM1G regulates protein translation and cell growth by dephosphorylating 4E-BP1

Abstract

Abstract Protein translation initiation is a tightly controlled process responding to nutrient availability and mitogen stimulation. Deregulation of translation initiation leads to tumor development and progression. Serving as one of the most important negative regulators of protein translation, 4E binding protein 1 (4E-BP1) binds to translation initiation factor 4E and inhibits cap-dependent translation in a phosphorylation-dependent manner. Hyperphosphorylation of 4E-BP1, which leads to inactivation of 4E-BP1, has been linked to human cancers with increased malignancy. While it has been demonstrated previously that the phosphosphorylation of 4E-BP1 is controlled by mTOR in the mTOR complex 1 (mTORC1), the mechanism underlying the dephosphorylation of 4E-BP1 remains elusive. Here, we report the identification of PPM1G as the phosphatase of 4E-BP1 through an shRNA library screen of Ser/Thr protein phosphatases. Co-immunoprecipitation experiment reveals that PPM1G binds to 4E-BP1 in cells, and purified PPM1G dephosphorylates 4E-BP1 in vitro. Knockdown of PPM1G in 293E and colon cancer HCT116 and SW480 cells results in an increase in the phosphorylation of 4E-BP1 at both Thr37/46 and Ser65 sites. This PPM1G-mediated dephosphorylation of 4E-BP1 is specific as the phosphorylation of upstream regulators of 4E-BP1 including Akt and mTOR is not altered in PPM1G knockdown cells. Furthermore, the time course of 4E-BP1 dephosphorylation induced by amino acid starvation or mTOR inhibitor treatment is significantly slowed down in PPM1G knockdown cells. Functionally, the amount of 4E-BP1 bound to the cap-dependent translation initiation complex is decreased when expression of PPM1G is depleted. As the result, the rate of cap-dependent translation, cell size, and protein content are increased in PPM1G knockdown cells. Taken together, our study has identified protein phosphatase PPM1G as a novel regulator of protein translation by negatively controlling the phosphorylation of 4E-BP1. Thus, PPM1G may serve as a tumor suppressor by directly counteracting mTOR to inhibit cell growth in cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4195. doi:1538-7445.AM2012-4195