Human Eukaryotic Release Factor 3a Depletion Causes Cell Cycle Arrest at G₁ Phase through Inhibition of the mTOR Pathway

Abstract

Eukaryotic release factor 3 (eRF3) is a GTPase associated with eRF1 in a complex that mediates translation termination in eukaryotes. Studies have related eRF3 with cell cycle regulation, cytoskeleton organization, and tumorigenesis. In mammals, two genes encode two distinct forms of eRF3, eRF3a and eRF3b, which differ in their N-terminal domains. eRF3a is the major factor acting in translation termination, and its expression level controls termination complex formation. Here, we investigate the role of eRF3a in cell cycle progression using short interfering RNAs and flow cytometry. We show that eRF3a depletion induces a G₁ arrest and that eRF3a GTP-binding activity, but not the eRF3a N-terminal domain, is required to restore G₁-to-S-phase progression. We also show that eRF3a depletion decreases the global translation rate and reduces the polysome charge of mRNA. Finally, we show that two substrates of the mammalian TOR (mTOR) kinase, 4E-BP1 and protein kinase S6K1, are hypophosphorylated in eRF3a-depleted cells. These results strongly suggest that the G₁ arrest and the decrease in translation induced by eRF3a depletion are due to the inhibition of mTOR activity and hence that eRF3a belongs to the regulatory pathway of mTOR activity.

This work was supported by the Association Française contre les Myopathies and by the Association pour la Recherche sur le Cancer (grant no. 3784). Céline Chauvin held fellowships from the French Ministère de la Recherche et de l'Enseignement Supérieur, from the Fondation pour la Recherche Médicale, and from the Société Française du Cancer.

We thank Michel Kress, Sophie Bellanger, and Olivier Ganier for their help in videomicroscopy and cell cycle analyses. We thank Isabelle Dusanter-Fourt and Mario Pende for the generous gift of the antibodies directed against proteins involved in the mTOR signaling pathway.