Translational Control of Programmed Cell Death: Eukaryotic Translation Initiation Factor 4E Blocks Apoptosis in Growth-Factor-Restricted Fibroblasts with Physiologically Expressed or Deregulated Myc

Abstract

There is increasing evidence that cell cycle transit is potentially lethal, with survival depending on the activation of metabolic pathways which block apoptosis. However, the identities of those pathways coupling cell cycle transit to survival remain undefined. Here we show that the eukaryotic translation initiation factor 4E (eIF4E) can mediate both proliferative and survival signaling. Overexpression of eIF4E completely substituted for serum or individual growth factors in preserving the viability of established NIH 3T3 fibroblasts. An eIF4E mutant (Ser-53 changed to Ala) defective in mediating its growth-factor-regulated functions was also defective in its survival signaling. Survival signaling by enforced expression of eIF4E did not result from autocrine release of survival factors, nor did it lead to increased expression of the apoptosis antagonists Bcl-2 and Bcl-X L . In addition, the execution apparatus of the apoptotic response in eIF4E-overexpressing cells was found to be intact. Increased expression of eIF4E was sufficient to inhibit apoptosis in serum-restricted primary fibroblasts with enforced expression of Myc. In contrast, activation of Ha-Ras, which is required for eIF4E proliferative signaling, did not suppress Myc-induced apoptosis. These data suggest that the eIF4E-activated pathways leading to survival and cell cycle progression are distinct. This dual signaling of proliferation and survival might be the basis for the potency of eIF4E as an inducer of neoplastic transformation.Normal diploid fibroblasts deprived of serum or polypeptide growth factors exit the cell cycle and persist in a quiescent state. When quiescent cells are stimulated to enter the cell cycle by viral oncogenes (31) or by enforced expression of cellular growth-promoting proteins such as c-Myc (2, 8; reviewed in reference 38), E2F (1, 17, 68), or cyclin A (18, 35), they do not progress toward mitosis but undergo programmed cell death. Similarly, the relaxation of negative growth control that occurs during preneoplastic progression of fibroblasts toward immortalization (42, 62) or by decreased retinoblastoma protein expression (1, 57) leads to apoptosis in response to growth factor withdrawal. Along these lines, peptide growth factors such as platelet-derived growth factor (PDGF) and epidermal growth factor can signal both cell cycle entry and apoptosis in fibroblasts deprived of progression-type growth factors (24). These findings support the idea that cell cycle transit is a potentially lethal condition unless specific rescue factors prevent this fate (20,43,44). Therefore, a mitogenic stimulus must coordinately activate metabolic pathways directing both proliferation and survival in order to result in cell division without death. While recent advances in our understanding of the cell division cycle reveal how cells integrate proliferative signals with the cell cycle machinery, the molecular mechanisms and regulatory pathways involved in growth factor survival signaling remain uncertain.The quiescent state is characterize...

Inhibition of Myc-dependent apoptosis by eukaryotic translation initiation factor 4E requires cyclin D1

“…In most cases, overexpression of eIF4E nearly eliminated apoptosis. These data extend our prior ®ndings that eIF4E suppresses apoptosis in response to serum restriction (Polunovsky et al, 1996), and indicate that eIF4E robustly suppresses apoptosis in response to both genotoxic and non-genotoxic cytostatic drugs.…”

“…Such cells are highly susceptible to apoptosis following serum deprivation or incubation with cytostatic agents (Evan et al, 1992;Harrington et al, 1994). We have previously determined that the mRNA cap binding protein, eIF4E, blocks apoptosis in growth factor restricted ®broblasts expressing physiologically regulated or deregulated c-Myc (Polunovsky et al, 1996). Here we report that rescue from cell cycle-active cytostatic drugs by ectopic eIF4E expression results from increased levels of cyclin D1.…”

“…Rescue of cycling cells by growth factors, at least in part, occurs through the PI3/Akt kinase pathway, which can operate independently of translational control (Datta et al, 1997;del Peso et al, 1997;Franke et al, 1997;Gonzalez-Garcia et al, 1997;Kennedy et al, 1997). We have reported a growth factor activated anti-apoptotic mechanism which operates through translation initiation factor 4E (eIF4E) (Polunovsky et al, 1996). A 24 kD polypeptide, eIF4E interacts with the cap structure (m 7 GpppX, where X is any nucleotide) of eukaryotic mRNA directing the translation machinery to the 5' end of most mRNA (reviewed in Sonenberg, 1996;Sonenberg and Gingras, 1998).…”

“…DNA histograms were analysed using Cell Quest software and cells with hypodiploid DNA content were de®ned as apoptotic. For each experimental condition, apoptosis was con®rmed using TUNEL, acridine orange staining, or ultrastructural analysis as previously described (Polunovsky et al, 1993(Polunovsky et al, , 1994(Polunovsky et al, , 1996.…”

“…Cells were ®xed with 70% ethanol (1 h, 48C) and stained with acridine orange. Apoptotic cells were quantitated morphologically by¯uorescence microscopy using previously established criteria (Polunovsky et al, 1996). At least 500 cells were counted per well and four independent experiments were performed.…”