Translational Repression of MCL-1 Couples Stress-induced eIF2α Phosphorylation to Mitochondrial Apoptosis Initiation

Abstract

The integrated stress response (ISR) integrates a broad range of environmental and endogenous stress signals to the phosphorylation of the α-subunit of eukaryotic translation initiation factor 2 (eIF2α). Although intense or prolonged activation of this pathway is known to induce apoptosis, the molecular mechanisms coupling stress-induced eIF2α phosphorylation to the cell death machinery have remained incompletely understood. In this study, we characterized apoptosis initiation in response to classical activators of the ISR (tunicamycin, UVC, elevated osmotic pressure, arsenite). We found that all applied stress stimuli activated a mitochondrial pathway of apoptosis initiation. Rapid and selective down-regulation of the anti-apoptotic BCL-2 family protein MCL-1 preceded the activation of BAX, BAK, and caspases. Stabilization of MCL-1 blocked apoptosis initiation, while cells with reduced MCL-1 protein content were strongly sensitized to stress-induced apoptosis. Stress-induced elimination of MCL-1 occurred with unchanged protein turnover and independently of MCL-1 mRNA levels. In contrast, stress-induced phosphorylation of eIF2α at Ser51 was both essential and sufficient for the down-regulation of MCL-1 protein in stressed cells. These findings indicate that stress-induced phosphorylation of eIF2α is directly coupled to mitochondrial apoptosis regulation via translational repression of MCL-1. Down-regulation of MCL-1 enables but not enforces apoptosis initiation in stressed cells. The integrated stress response (ISR) integrates a broad range of environmental and endogenous stress signals to the phosphorylation of the α-subunit of eukaryotic translation initiation factor 2 (eIF2α). Although intense or prolonged activation of this pathway is known to induce apoptosis, the molecular mechanisms coupling stress-induced eIF2α phosphorylation to the cell death machinery have remained incompletely understood. In this study, we characterized apoptosis initiation in response to classical activators of the ISR (tunicamycin, UVC, elevated osmotic pressure, arsenite). We found that all applied stress stimuli activated a mitochondrial pathway of apoptosis initiation. Rapid and selective down-regulation of the anti-apoptotic BCL-2 family protein MCL-1 preceded the activation of BAX, BAK, and caspases. Stabilization of MCL-1 blocked apoptosis initiation, while cells with reduced MCL-1 protein content were strongly sensitized to stress-induced apoptosis. Stress-induced elimination of MCL-1 occurred with unchanged protein turnover and independently of MCL-1 mRNA levels. In contrast, stress-induced phosphorylation of eIF2α at Ser51 was both essential and sufficient for the down-regulation of MCL-1 protein in stressed cells. These findings indicate that stress-induced phosphorylation of eIF2α is directly coupled to mitochondrial apoptosis regulation via translational repression of MCL-1. Down-regulation of MCL-1 enables but not enforces apoptosis initiation in stressed cells. The integrated stress response (ISR) 2The abbreviations used are: ISR, integrated stress response; eIF2α, α-subunit of eukaryotic translation initiation factor 2; CHAPS, 3-[(3-cholamidopropy-l)dimethylammonio]-1-propanesulfonic acid; Z, benzyloxycarbonyl; fmk, fluoromethylketone. is a general stress response program conserved from yeast to mammals, that is known to integrate various types of environmental and endogenous stress signals, including endoplasmic reticulum stress, amino acid deprivation, infection with double-stranded RNA viruses, osmotic stress, UV light exposure, heme deficiency, and oxidative stress (1Ron D. J. Clin. Investig. 2002; 110: 1383-1388Crossref PubMed Scopus (745) Google Scholar, 2Zhang P. McGrath B.C. Reinert J. Olsen D.S. Lei L. Gill S. Wek S.A. Vattem K.M. Wek R.C. Kimball S.R. Jefferson L.S. Cavener D.R. Mol. Cell. Biol. 2002; 22: 6681-6688Crossref PubMed Scopus (352) Google Scholar, 3Deng J. Harding H.P. Raught B. Gingras A.C. Berlanga J.J. Scheuner D. Kaufman R.J. Ron D. Sonenberg N. Curr. Biol. 2002; 12: 1279-1286Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar, 4Lu L. Han A.P. Chen J.J. Mol. Cell. Biol. 2001; 21: 7971-7980Crossref PubMed Scopus (242) Google Scholar). Those diverse signals activate specific stress kinases, each of which converges on the phosphorylation at Ser51 on the α-subunit of eukaryotic translation initiation factor 2 (eIF2α) (1Ron D. J. Clin. Investig. 2002; 110: 1383-1388Crossref PubMed Scopus (745) Google Scholar). Phosphorylation of eIF2α at Ser51 abrogates the function of eIF2α, required for the transfer of the initiator Met-tRNA Meti to the small ribosomal subunit. This leads to a shutdown of global mRNA translation due to reduced AUG initiator codon recognition, along with increased translation of a few selected mRNAs including ATF-4, a basic zipper transcription activator (5Holcik M. Sonenberg N. Nat. Rev. Mol. Cell. Biol. 2005; 6: 318-327Crossref PubMed Scopus (1046) Google Scholar, 6Harding H.P. Novoa I. Zhang Y. Zeng H. Wek R. Schapira M. Ron D. Mol. Cell. 2000; 6: 1099-1108Abstract Full Text Full Text PDF PubMed Scopus (2431) Google Scholar, 7Vattem K.M. Wek R.C. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 11269-11274Crossref PubMed Scopus (1148) Google Scholar). As a consequence of global translational arrest, the steady-state levels of most cellular proteins decrease with time, dependent on their respective protein half-life. This has recently been found to be essential for NF-κB activation mediated by down-regulation of IκB proteins in response to eIF2α phosphorylation (8Jiang H.Y. Wek R.C. Biochem. J. 2005; 385: 371-380Crossref PubMed Scopus (92) Google Scholar, 9Jiang H.Y. Wek S.A. McGrath B.C. Scheuner D. Kaufman R.J. Cavener D.R. Wek R.C. Mol. Cell. Biol. 2003; 23: 5651-5663Crossref PubMed Scopus (358) Google Scholar, 10Deng J. Lu P.D. Zhang Y. Scheuner D. Kaufman R.J. Sonenberg N. Harding H.P. Ron D. Mol. Cell. Biol. 2004; 24: 10161-10168Crossref PubMed Scopus (524) Google Scholar). These findings provide a first striking example of a signaling pathway activated by translational repression of a labile protein during eIF2α phosphorylation. Activation of the ISR also mobilizes stress-induced gene expression induced by ATF4 and its target genes, involved in cell growth, differentiation and apoptosis (11Harding H.P. Zhang Y. Zeng H. Novoa I. Lu P.D. Calfon M. Sadri N. Yun C. Popko B. Paules R. Stojdl D.F. Bell J.C. Hettmann T. Leiden J.M. Ron D. Mol. Cell. 2003; 11: 619-633Abstract Full Text Full Text PDF PubMed Scopus (2410) Google Scholar, 12Jiang H.Y. Wek S.A. McGrath B.C. Lu D. Hai T. Harding H.P. Wang X. Ron D. Cavener D.R. Wek R.C. Mol. Cell. Biol. 2004; 24: 1365-1377Crossref PubMed Scopus (393) Google Scholar). The biological consequences of activation of the ISR largely depend on the context. While activation of the eIF2α phosphorylation pathway is primarily thought to protect cells from ongoing damage (11Harding H.P. Zhang Y. Zeng H. Novoa I. Lu P.D. Calfon M. Sadri N. Yun C. Popko B. Paules R. Stojdl D.F. Bell J.C. Hettmann T. Leiden J.M. Ron D. Mol. Cell. 2003; 11: 619-633Abstract Full Text Full Text PDF PubMed Scopus (2410) Google Scholar, 13Lu P.D. Jousse C. Marciniak S.J. Zhang Y. Novoa I. Scheuner D. Kaufman R.J. Ron D. Harding H.P. EMBO J. 2004; 23: 169-179Crossref PubMed Scopus (308) Google Scholar, 14Harding H.P. Zhang Y. Bertolotti A. Zeng H. Ron D. Mol. Cell. 2000; 5: 897-904Abstract Full Text Full Text PDF PubMed Scopus (1570) Google Scholar), prolonged or extensive activation of this pathway has been found to induce apoptosis (15Scheuner D. Patel R. Wang F. Lee K. Kumar K. Wu J. Nilsson A. Karin M. Kaufman R.J. J. Biol. Chem. 2006; 281: 21458-21468Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar, 16Jiang H.Y. Wek R.C. J. Biol. Chem. 2005; 280: 14189-14202Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar, 17Kazemi S. Papadopoulou S. Li S. Su Q. Wang S. Yoshimura A. Matlashewski G. Dever T.E. Koromilas A.E. Mol. Cell. Biol. 2004; 24: 3415-3429Crossref PubMed Scopus (85) Google Scholar, 18Gil J. Alcami J. Esteban M. Mol. Cell. Biol. 1999; 19: 4653-4663Crossref PubMed Google Scholar). However, the molecular mechanisms coupling activation of the ISR to the initiation of apoptosis have remained incompletely understood. In the vast majority of cell types, apoptosis depends on the mitochondrial pathway. Therein, disruption of mitochondrial integrity leads to the release of pro-apoptotic proteins such as cytochrome c, AIF, Smac/Diablo, EndoG, and HtrA2/Omi from the mitochondrial inter-membrane space into the cytosol (19Green D.R. Cell. 2005; 121: 671-674Abstract Full Text Full Text PDF PubMed Scopus (667) Google Scholar). Cytochrome c contacts Apaf-1, thereby triggering the recruitment and auto-activation of procaspase-9. Caspase-9 then activates a cascade of specialized proteases, termed executioner caspases, which in turn execute cell death. Mitochondrial integrity is regulated by the members of the BCL-2 family of proteins (20Cory S. Adams J.M. Nat. Rev. Cancer. 2002; 2: 647-656Crossref PubMed Scopus (3349) Google Scholar). BAX and BAK, two pro-apoptotic multidomain BCL-2 proteins, directly or indirectly induce mitochondrial outer membrane permeabilization (MOMP) by oligomerization occurring upon their activation (21Wei M.C. Zong W.X. Cheng E.H. Lindsten T. Panoutsakopoulou V. Ross A.J. Roth K.A. MacGregor G.R. Thompson C.B. Korsmeyer S.J. Science. 2001; 292: 727-730Crossref PubMed Scopus (3373) Google Scholar). MOMP is prevented by multidomain anti-apoptotic BCL-2 proteins such as BCL-2, BCL-xL, and MCL-1. Members of the BH3-only group of BCL-2 proteins undergo induction and/or activation by apoptotic stimuli and either directly activate BAX-BAK or specifically neutralize anti-apoptotic multidomain BCL-2 proteins (22Kim H. Rafiuddin-Shah M. Tu H.C. Jeffers J.R. Zambetti G.P. Hsieh J.J. Cheng E.H. Nat. Cell Biol. 2006; 8: 1348-1358Crossref PubMed Scopus (707) Google Scholar). In this study, we aimed to uncover the molecular cross-talk between cellular stress signaling and apoptosis. We characterized apoptosis initiation in response to classical activators of the ISR and found that stress-induced apoptosis essentially involves the rapid and selective down-regulation of MCL-1 protein, mediated by stress-induced phosphorylation of eIF2α at Ser51. Materials—The following drugs were purchased as indicated: Coumermycin (Fluka); ZVAD-fmk and and were in and in cell and were the Cell and and cells were in with 2 and and cells were in cells were to a in a at UV was and cells were with a small of The of the cell was and cells were to a osmotic stress, cells were to cell with in for and ZVAD-fmk were to cells to the to cellular and were into cells to the were at a of and used for expression of in the expression was to and were in the of were for expression by MCL-1 RNA specifically MCL-1 were the and by at a of were with following the were of and was by a from cells. was at the of MCL-1 was by of from into were by of the respective eIF2α from expression by was at the of the respective eIF2α and the respective were by of from expression by Dever and C. Lu J. K. Dever T.E. EMBO J. 2001; PubMed Scopus Google Scholar). of all was by and on and cells were in with and and was as recently G. D. R. EMBO J. 2006; PubMed Scopus Google Scholar). of was from the used for for and BAX were of cells in CHAPS, protein was with of for 2 of or were to of cell and at were with for 2 and were with proteins were by and to BAX or were by as RNA was from cells the was in all RNA to mRNAs were by as G. D. R. EMBO J. 2006; PubMed Scopus Google Scholar). on of specifically target were that not the mRNAs or of mRNA were by by of the respective target as of were from at were by of with to the each a of from were Mitochondrial of in to of the apoptosis initiation in response to activation of the ISR, cells were to a of classical ISR the elevated osmotic and stress stimuli were applied at found to a apoptotic response in cells. Activation of BAX and was by L. J. C. J. Cell Biol. 1999; PubMed Scopus Google Scholar, R.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). of and was by applied stress stimuli induced the activation of BAX and as as of and of apoptosis were by such as and of not were in the of a of ZVAD-fmk strongly of and prevented of as as of However, of ZVAD-fmk not the activation of BAX and BAK, that BAX and were activated of and that apoptosis was at the Down-regulation of MCL-1 in to integrity is by the of and anti-apoptotic members of the BCL-2 protein uncover the levels of and anti-apoptotic multidomain BCL-2 proteins during apoptosis initiation in response to the applied ISR we for BAX, BAK, BCL-2, BCL-xL, and MCL-1 all applied stress stimuli induced the rapid and selective down-regulation of the anti-apoptotic BCL-2 protein MCL-1. Down-regulation of MCL-1 preceded all of apoptosis initiation and was not by of ZVAD-fmk However, the decrease in MCL-1 levels remained by of MCL-1 occurred in response to osmotic stress, and was blocked Down-regulation of MCL-1 in to to a Cell that the rapid down-regulation of MCL-1 in response to the applied stress stimuli is not to a cell we cell to UVC, elevated osmotic pressure, and and MCL-1 protein levels. Cell were and cells as as and cells. As in MCL-1 was in response to all of the applied in all cell Stabilization of MCL-1 by Stress-induced aimed to MCL-1 stress-induced apoptosis. MCL-1 is a turnover protein that is by the Q. F. Wang X. Cell. 2005; 121: Full Text Full Text PDF PubMed Scopus Google Scholar). of a but not prevented the decrease in MCL-1 levels in response to UVC, osmotic stress and and blocked mitochondrial apoptosis initiation as by of BAX and of of the and thereby of a of proteins, we of apoptosis by was dependent on the of MCL-1. this we cells either with or specifically MCL-1 also and to the respective in the of As in 2 and blocked apoptosis initiation in but not in cells with strongly reduced MCL-1 protein content These findings indicate that of MCL-1 at levels for the of apoptosis initiation in the of of not induce apoptosis during the of in both and cells with reduced MCL-1 However, occurred cells were to for we not with that required to induce is a of apoptosis in response to activators of the cells were either or were with or with each of two specifically MCL-1 were for MCL-1 protein content by cells and cells with or specifically MCL-1 were to of UVC, osmotic stress, and as were with osmotic stress, or was by cells with or were to of UVC, osmotic stress, and as was by as MCL-1 a in to to anti-apoptotic BCL-2 proteins and BCL-2, MCL-1 is a protein that regulation at the of and J. G. J. Biochem. Cell Biol. 2005; PubMed Scopus Google Scholar). We aimed to in MCL-1 protein levels the cellular response to the applied ISR this we MCL-1 protein expression RNA and In apoptosis was by of and for of MCL-1 protein of cells with two specifically MCL-1. In with findings by D. M. Q. F. Wang X. 2003; PubMed Scopus Google Scholar), down-regulation of MCL-1 protein was not sufficient to induce cell death and cells with reduced MCL-1 protein levels and not However, cells with reduced MCL-1 protein expression levels were strongly sensitized to the applied stress stimuli and apoptosis in response to stress and These findings indicate that MCL-1 protein levels apoptosis MCL-1 for the molecular mechanisms down-regulation of MCL-1 in response to the applied activators of the the was used of As in elimination of MCL-1 in response to the applied occurred this was due to in MCL-1 protein the of MCL-1 protein were by cells with the translation the time, cells were either to or to stress stimuli as of a MCL-1 protein of in cells of the applied the of MCL-1 protein with and These findings that the rapid decrease in MCL-1 levels in response to cellular stress is not mediated by MCL-1 protein of on MCL-1 MCL-1 mRNA in response to the applied stress As in on MCL-1 mRNA was of cells to UV light induced a rapid and of MCL-1 while osmotic stress a in MCL-1 mRNA by levels not MCL-1 mRNA induced a but in MCL-1 mRNA levels were in with cells not MCL-1 mRNA for Down-regulation of MCL-1 rapid decrease of MCL-1 mRNA in response to the of MCL-1 protein down-regulation of MCL-1 mRNA was required for the down-regulation of MCL-1 protein, we characterized of MCL-1 As in induced a in MCL-1 mRNA levels. of cells with to mRNA a MCL-1 mRNA of of cells to strongly MCL-1 mRNA by a of the that UV light with MCL-1 mRNA of mRNA during cellular stress has been found to depend on the and of eukaryotic mRNAs N. P. Biochem. 2002; PubMed Scopus Google Scholar). we MCL-1 mRNA of and undergo cell and with were to UV light and both mRNA and protein levels of MCL-1 and endogenous MCL-1 were As in endogenous MCL-1 mRNA was in response to UVC, while mRNA remained largely However, at protein both MCL-1 and endogenous MCL-1 protein were with that the down-regulation of MCL-1 protein occurred independently of MCL-1 mRNA In all of the applied MCL-1 protein of MCL-1 mRNA and of MCL-1 protein at the of mRNA Phosphorylation of eIF2α the of MCL-1 have been to induce global translational by the phosphorylation of the α-subunit of eukaryotic translation initiation factor 2 (eIF2α) at to of the required for the of Met-tRNA Meti to the ribosomal (5Holcik M. Sonenberg N. Nat. Rev. Mol. Cell. Biol. 2005; 6: 318-327Crossref PubMed Scopus (1046) Google Scholar). stress-induced down-regulation of MCL-1 was to eIF2α the of eIF2α phosphorylation and MCL-1 down-regulation were in As in all of the applied induced a phosphorylation of eIF2α that was by the down-regulation of MCL-1. In of cells to induced a and eIF2α phosphorylation that with MCL-1 protein levels stress induced a but phosphorylation of eIF2α, which was by a but decrease of MCL-1 protein the and ISR activator and induced eIF2α phosphorylation and down-regulation of MCL-1 protein in Phosphorylation of eIF2α for the Down-regulation of MCL-1 in phosphorylation of eIF2α at Ser51 is essential for the down-regulation of MCL-1 in response to activators of the ISR, we a eIF2α in cells. of has been to the eIF2α phosphorylation pathway and ongoing mRNA translation during cellular stress C. C. M. S. A. Sonenberg N. Koromilas A. Mol. Cell. Biol. 2002; 22: PubMed Scopus Google Scholar). were with MCL-1 protein specifically in we cells with expression for to osmotic stress and was in cells eIF2α and but not in cells that phosphorylation of eIF2α is essential for the down-regulation of MCL-1 of a eIF2α to MCL-1 phosphorylation of eIF2α at Ser51 is sufficient to MCL-1 we eIF2α phosphorylation of eIF2α in and endogenous MCL-1 protein levels. the respective eIF2α by eIF2α phosphorylation and not by endogenous eIF2α phosphorylation Mol. Cell. Biol. 2004; 24: PubMed Scopus Google Scholar), which was not in As in expression of the MCL-1 protein expression while expression of the induced a in MCL-1 protein levels with cells with or In contrast, MCL-1 mRNA levels remained by mRNA was in all while protein expression was reduced in cells with that the of translation induced by also with its protein in cells not to of occurring cells. were with cells with and the respective eIF2α In expression of the protein and with cells and 2 and and Activation of the of phosphorylation of eIF2α by of its in elimination of we used a the of the protein and C. Lu J. K. Dever T.E. EMBO J. 2001; PubMed Scopus Google Scholar). is of known to eIF2α at Ser51 R.C. H.Y. Biochem. 2006; PubMed Scopus Google Scholar). The protein rapid and activation upon of As in cells were with expression for and and or the and of induced in eIF2α phosphorylation as as a of MCL-1 protein in cells but not in or in cells activation of to a decrease of its protein expression of the findings with expression of in cells was were with cells with and the respective As in of induced a elimination of by increased eIF2α phosphorylation specifically in cells and to and 2 and and that phosphorylation of eIF2α is sufficient to and MCL-1 Down-regulation of MCL-1 but in this study, we that apoptosis initiation in cells with stimuli the integrated stress response involves the rapid down-regulation of the anti-apoptotic BCL-2 family protein MCL-1. of MCL-1 has been to be required for apoptosis induction in response to a of stimuli including D. M. Q. F. Wang X. 2003; PubMed Scopus Google Scholar), infection A. C. D. 2003; PubMed Scopus Google Scholar), E.H. H. A. B. Thompson C.B. Mol. Cell. Biol. PubMed Scopus Google Scholar), and U. C. D.R. Mol. Cell. 2006; 21: Full Text Full Text PDF PubMed Scopus Google Scholar). findings to a of MCL-1 in the of apoptosis in stressed cells. In we found that down-regulation of MCL-1 was sufficient to strongly cells to stress that MCL-1 protein levels a in cell This is of MCL-1 is the group of anti-apoptotic members of the family in that is a regulated protein to and signaling J. G. J. Biochem. Cell Biol. 2005; PubMed Scopus Google Scholar). In with D. M. Q. F. Wang X. 2003; PubMed Scopus Google Scholar, Chen L. G. A. Adams J.M. 2005; 19: PubMed Scopus Google Scholar), we found that down-regulation of MCL-1 was not sufficient to induce apoptosis. These that apoptotic with down-regulation of MCL-1 to mitochondrial integrity in stressed cells. Activation of has been found to be involved in apoptosis K. R.J. Proc. Natl. Acad. Sci. U. S. A. 2003; PubMed Scopus Google and apoptosis induction in response to P. D. F. Adams J.M. A. Science. 1999; PubMed Scopus Google Scholar), a known to induce have found induction of and to be involved in apoptosis J. Lee B. Lee J. Biol. Chem. 2006; 281: Full Text Full Text PDF PubMed Scopus Google Scholar, C. D. A. T. J. Cell Biol. 2003; PubMed Scopus Google Scholar). be required to apoptotic involved and to signals in a or with activation of the integrated stress Down-regulation of MCL-1 the of Stress-induced Phosphorylation of that down-regulation of MCL-1 in stressed cells is the consequence of stress-induced phosphorylation of eIF2α at Ser51. This is by the following of the applied stress stimuli MCL-1 protein and decrease of MCL-1 mRNA occurred in response to and was for down-regulation of MCL-1 Phosphorylation of eIF2α at Ser51 the decrease in MCL-1 and of this pathway prevented decrease of MCL-1 in stressed cells. eIF2α phosphorylation and phosphorylation of eIF2α by in the of stress stimuli was sufficient to MCL-1. mechanisms have been to to a down-regulation of MCL-1 during apoptosis. In response to elimination of MCL-1 occurred with unchanged protein turnover due to a of MCL-1 protein D. M. Q. F. Wang X. 2003; PubMed Scopus Google Scholar). have found that with MCL-1 Chen L. G. A. Adams J.M. 2005; 19: PubMed Scopus Google Scholar, Chen L. S. Adams J.M. Cell. 2006; Full Text Full Text PDF PubMed Scopus Google Scholar). of MCL-1 occurred in response to phosphorylation by U. C. D.R. Mol. Cell. 2006; 21: Full Text Full Text PDF PubMed Scopus Google Scholar). a MCL-1 protein is in stressed cells. on the phosphorylation of eIF2α at to a global shutdown of mRNA all mRNAs with a few (5Holcik M. Sonenberg N. Nat. Rev. Mol. Cell. Biol. 2005; 6: 318-327Crossref PubMed Scopus (1046) Google Scholar). expression levels of most cellular proteins decrease with time, to their respective protein The rapid decrease in the steady-state of proteins has recently been found to to the activation of NF-κB in response to eIF2α mediated by the down-regulation of IκB proteins (8Jiang H.Y. Wek R.C. Biochem. J. 2005; 385: 371-380Crossref PubMed Scopus (92) Google Scholar, 9Jiang H.Y. Wek S.A. McGrath B.C. Scheuner D. Kaufman R.J. Cavener D.R. Wek R.C. Mol. Cell. Biol. 2003; 23: 5651-5663Crossref PubMed Scopus (358) Google Scholar, 10Deng J. Lu P.D. Zhang Y. Scheuner D. Kaufman R.J. Sonenberg N. Harding H.P. Ron D. Mol. Cell. Biol. 2004; 24: 10161-10168Crossref PubMed Scopus (524) Google Scholar). The down-regulation of MCL-1 protein in response to eIF2α phosphorylation the basic The decrease of MCL-1 its protein turnover Q. F. Wang X. Cell. 2005; 121: Full Text Full Text PDF PubMed Scopus Google a of protein to a of MCL-1 the global of mRNA translation from eIF2α phosphorylation in stressed cells specific on mitochondrial apoptosis signaling by as essential of apoptosis initiation in stressed cells. Down-regulation of MCL-1 to of the ISR has been found to death or on the P.D. Jousse C. Marciniak S.J. Zhang Y. Novoa I. Scheuner D. Kaufman R.J. Ron D. Harding H.P. EMBO J. 2004; 23: 169-179Crossref PubMed Scopus (308) Google Scholar, D. Patel R. Wang F. Lee K. Kumar K. Wu J. Nilsson A. Karin M. Kaufman R.J. J. Biol. Chem. 2006; 281: 21458-21468Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar). While the of the ISR has been found to death induction of the ATF4 target H. M. Wang X. N. H. Ron D. 12: PubMed Scopus Google Scholar, S.J. Yun S. Novoa I. Zhang Y. R. K. Harding H.P. Ron D. 2004; PubMed Scopus Google Scholar), the translational by eIF2α has been a pathway (11Harding H.P. Zhang Y. Zeng H. Novoa I. Lu P.D. Calfon M. Sadri N. Yun C. Popko B. Paules R. Stojdl D.F. Bell J.C. Hettmann T. Leiden J.M. Ron D. Mol. Cell. 2003; 11: 619-633Abstract Full Text Full Text PDF PubMed Scopus (2410) Google Scholar). This has been by that eIF2α phosphorylation also as cellular death (15Scheuner D. Patel R. Wang F. Lee K. Kumar K. Wu J. Nilsson A. Karin M. Kaufman R.J. J. Biol. Chem. 2006; 281: 21458-21468Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar, 16Jiang H.Y. Wek R.C. J. Biol. Chem. 2005; 280: 14189-14202Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar, 17Kazemi S. Papadopoulou S. Li S. Su Q. Wang S. Yoshimura A. Matlashewski G. Dever T.E. Koromilas A.E. Mol. Cell. Biol. 2004; 24: 3415-3429Crossref PubMed Scopus (85) Google Scholar, 18Gil J. Alcami J. Esteban M. Mol. Cell. Biol. 1999; 19: 4653-4663Crossref PubMed Google Scholar), the of eIF2α phosphorylation to cell death. findings that phosphorylation of eIF2α is coupled to the rapid down-regulation of the anti-apoptotic protein MCL-1. We found that this decrease of MCL-1 is required for apoptosis initiation in stressed cells. However, down-regulation of MCL-1 is not sufficient to mitochondrial phosphorylation of eIF2α not induce cell this pathway with a and a of eIF2α on occurring signaling be required to the of death and activated by eIF2α phosphorylation in stressed of the or death is We Ron for eIF2α expression and Dever for with