Translation of terminal oligopyrimidine tract (TOP) mRNAs, which encode multiple components of the protein synthesis machinery, is known to be controlled by mitogenic stimuli. We now show that the ability of cells to progress through the cell cycle is not a prerequisite for this mode of regulation. TOP mRNAs can be translationally activated when PC12 or embryonic stem (ES) cells are induced to grow (increase their size) by nerve growth factor and retinoic acid, respectively, while remaining mitotically arrested. However, both growth and mitogenic signals converge via the phosphatidylinositol 3-kinase (PI3-kinase)-mediated pathway and are transduced to efficiently translate TOP mRNAs. Translational activation of TOP mRNAs can be abolished by LY294002, a PI3-kinase inhibitor, or by overexpression of PTEN as well as by dominant-negative mutants of PI3-kinase or its effectors, PDK1 and protein kinase B␣ (PKB␣). Likewise, overexpression of constitutively active PI3-kinase or PKB␣ can relieve the translational repression of TOP mRNAs in quiescent cells. Both mitogenic and growth signals lead to phosphorylation of ribosomal protein S6 (rpS6), which precedes the translational activation of TOP mRNAs. Nevertheless, neither rpS6 phosphorylation nor its kinase, S6K1, is essential for the translational response of these mRNAs. Thus, TOP mRNAs can be translationally activated by growth or mitogenic stimuli of ES cells, whose rpS6 is constitutively unphosphorylated due to the disruption of both alleles of S6K1. Similarly, complete inhibition of mammalian target of rapamycin (mTOR) and its effector S6K by rapamycin in various cell lines has only a mild repressive effect on the translation of TOP mRNAs. It therefore appears that translation of TOP mRNAs is primarily regulated by growth and mitogenic cues through the PI3-kinase pathway, with a minor role, if any, for the mTOR pathway.Cell proliferation involves two processes: cell growth (increase in cell size) and cell division, which are normally intermingled, to the extent that cells must attain a minimal size to progress in the cell cycle. The dependence of DNA replication and cell division on cellular growth appears to enable accumulation of cellular resources to ensure daughter cell survival. Growth is characterized by elevated production of the translational apparatus needed to cope with the increasing demand for protein synthesis (42). Indeed, according to one estimate, most of the energy consumed during cellular growth is utilized for generating the components of the protein synthesis machinery (53).TOP mRNAs, which encode many components of the translational apparatus [ribosomal proteins, elongation factors eEF1A and eEF2, and poly(A)-binding protein], are translationally regulated by mitogenic signals through their 5Ј terminal oligopyrimidine tract (5ЈTOP) (35). Translational repression of TOP mRNAs is apparent when proliferation of vertebrate cells is blocked by a wide variety of physiological signals (terminal differentiation, contact inhibition, and serum starvatio...