Abstract
Activated Ras has been shown to provide powerful antiapoptotic signals to cells through well defined transcriptional and post- translational pathways, whereas translational control as a mechanism of Ras survival signaling remains unexplored. Here we show a direct relationship between assembly of the cap-dependent translation initiation apparatus and suppression of apoptosis by oncogenic Ras in vitro and in vivo. Decreasing protein synthesis with rapamycin, which is known to inhibit cap-dependent translation, increases the susceptibility of Ras-transformed fibroblasts to cytostatic drug-induced apoptosis. In contrast, suppressing global protein synthesis with equipotent concentrations of cycloheximide actually prevents apoptosis. Enforced expression of the cap-dependent translational repressor, the eukaryotic translation initiation factor (eIF) 4E-binding protein (4E-BPI), sensitizes fibroblasts to apoptosis in a manner strictly dependent on its ability to sequester eIF4E from a translationally active complex with eIF4GI and the co-expression of oncogenic Ras. Ectopic expression of 4E-BP1 also promotes apoptosis of Ras-transformed cells injected into immunodeficient mice and markedly diminishes their tumorigenicity. These results establish that eIF4E-dependent protein synthesis is essential for survival of fibroblasts bearing oncogenic Ras and support the concept that activation of cap-dependent translation by extracellular ligands or intrinsic survival signaling molecules suppresses apoptosis, whereas synthesis of proteins mediating apoptosis can occur independently of the cap.
Highlights
When rapamycin was added to Ras-transformed cells, it caused a dose-dependent decline in protein synthesis, which paralleled its ability to sensitize cells to lovastatin-induced apoptosis (Fig. 1c)
Equipotent doses of the peptide elongation inhibitor cycloheximide blocked apoptosis (Fig. 1d), demonstrating that the execution of lovastatin-induced cell death requires global protein synthesis. These results suggest that a generalized inhibition of mRNA translation is not the means by which rapamycin exerts its proapoptotic effect, rather they point toward a selective inhibition of antiapoptotic mRNA translation or a mechanism independent of its ability to repress translation
Examples of selective control have emerged involving regulation at the translation initiation step, in the integration of pleiotropic responses leading to differentiation, FIG. 3. 4E-BP1-promoted apoptosis in Ras-transformed cells is associated with displacement of translation factor eIF4GI from eIF4E. a, immunoblot analysis of 4E-BP1 and eIF4GI associated with cap-bound eIF4E in clones of Cloned rat embryo fibroblasts (CREF)/RasV12 ectopically expressing wild type 4E-BP1. b and c, apoptosis is shown as a function of the 4EBP1/eIF4E (b) or eIF4G/eIF4E (c) ratio in the indicated CREF/Ras/BP1-wt clones incubated in growth medium for 24 h in the presence or absence of 5 M lovastatin. d, 4EBP1 lacking an eIF4E binding domain does not promote apoptosis
Summary
Enforced expression of the cap-dependent translational repressor, the eukaryotic translation initiation factor (eIF) 4Ebinding protein (4E-BPI), sensitizes fibroblasts to apoptosis in a manner strictly dependent on its ability to sequester eIF4E from a translationally active complex with eIF4GI and the co-expression of oncogenic Ras. Ectopic expression of 4E-BP1 promotes apoptosis of Ras-transformed cells injected into immunodeficient mice and markedly diminishes their tumorigenicity. To elucidate the role of translational control in Ras survival signaling, here we focus directly on the 5Ј-mRNA cap binding complex and examine the induction of apoptosis in cells transformed with oncogenic Ras after a generalized reduction in protein synthesis or after specific repression of cap-dependent translation initiation. Extracellular survival factors suppress the intrinsic apoptotic apparatus through cognate receptor kinases at the cell surface, which activate the proto-oncogene ras and a number of pleiotropic transcriptional and post-translational effector pathways [1]. Akt-mediated phosphorylation of the Bcl-2 family member Bad [6] and the cell death protease caspase-9 [7] is implicated in post-translational suppression of
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