Versatility of the Translational Machinery During Stress: Changing Partners to Keep Dancing

Versatility of the translational machinery during stress: changing partners to keep dancing

Fátima Gebauer1

1Gene Regulation, Stem Cells and Cancer Programme, Centre for Genomic Regulation (CRG-UPF), Dr Aiguader 88, 08003-Bar- celona, Spain Cell Research (2012) 22:1634-1636. doi:10.1038/cr.2012.102; published online 3 July 2012

Cap-dependent translation is initi- poxia [2]. Thus, by using a specialized unbiased search for HIF-2α interactors ated by the binding of eIF4E to the isoform of eIF4E in complex with a identified RBM4, a protein involved cap structure at the 5′ end of mRNAs. hypoxia-induced RNP, the translational in translational control [3, 4]. RBM4 During hypoxic stress, global transla- machinery preserves selective protein interacts with the HIF-2α-responsive tion decreases because eIF4E is inac- synthesis during stress (Figure 1). region of EGFR 3′ UTR, and its deple- tivated. In a recent article in Nature, The hypoxia inducible factor (HIF) tion results in dissociation of HIF-2α Lee and colleagues show that residual family of transcription factors maintain from EGFR mRNA, indicating that hypoxic translation is maintained by cellular oxygen homeostasis. HIF-2α is RBM4 recruits HIF-2α to the 3′ UTR a specialized isoform of eIF4E, which induced under hypoxia and activates the of the EGFR transcript. Furthermore, binds to target mRNAs in complex expression of epidermal growth factor depletion of RBM4 abrogates the hy- with a hypoxia-induced RNP. receptor (EGFR), a factor that confers poxic translation of endogenous EGFR Translation of most messenger RNAs growth advantage to hypoxic tumor or EGFR reporters without affecting the in the cell is initiated by binding of eu- cells. Lee and colleagues first show that levels of HIF-2α. Thus, RBM4 mediates karyotic initiation factor (eIF) 4E to the the accumulation of EGFR is indepen- the effects of HIF-2α in hypoxic transla- m7GpppN cap structure at the mRNA dent of the transcriptional activity of tion of EGFR mRNA. 5′ end. Multiple eIF4E isoforms have HIF-2α. First, EGFR protein accumu- But the HIF-2α/RBM4 complex been described in a variety of organ- lates in cells treated with transcription regulates more than just EGFR trans- isms along the evolutionary scale, but inhibitors; second, no changes in the lation. Silencing of HIF-2α or RBM4 their functions remain for the most part steady state amounts of EGFR mRNA considerably reduced the rate of global obscure [1]. Translation is drastically are detected upon hypoxia; and third, hypoxic translation as measured by 35S- reduced upon low oxygen tension (hy- depletion of HIF-1β, a factor required methionine incorporation into nascent poxia) by mechanisms that inhibit the for HIF transcriptional activity, has no proteins. UV crosslinking of RNAs function of eIF4E. Cell function under effect on EGFR expression. These re- associated with the HIF-2α/RBM4 hypoxia, however, is maintained by a sults pointed to a surprising direct role complex identified CGG or CG boxes significant degree of “residual” transla- of HIF-2α in EGFR mRNA translation, in hundreds of transcripts, a hallmark of tion. In a recent article in Nature, Lee a notion reinforced by the finding that RBM4 binding [5]. Consistent with the and colleagues show that the eIF4E HIF-2α associates with polysomes. relevance of the RBM4 site, mutation homologue 4E2, in complex with the The authors further show that HIF- of a specific CGG box in the 3′ UTR transcription factor HIF-2α and the 2α associates to a region of EGFR 3′ of EGRF abrogated hypoxia-induced RNA-binding protein RBM4, sustains UTR that is necessary and sufficient to translation of a reporter. The CGG, cap-dependent translation during hy- drive translation of reporters in a HIF- however, was not sufficient to confer 2α-dependent manner. HIF-2α does hypoxic translation, as it must be em- not contain discernible RNA-binding bedded in a region that the authors refer Correspondence: Fátima Gebauer motifs, raising the possibility that RNA to as the rHRE (RNA Hypoxia Response Tel: +34-93-3160120; Fax: +34-93-3969983 binding is mediated by interactions Element). E-mail: [email protected] with bridge RNA-binding proteins. An How does the HIF-2α/RBM4 com-

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Figure 1 Translational control under hypoxia. In normal growing conditions (normoxia, 21% O2), translation of the vast major- ity of transcripts is initiated via a mechanism where eIF4E binds to the cap structure (black circle) together with eIF4A and additional factors (eIF4G and PABP, gray ovals) to promote ribosome recruitment. In this condition, 4EBP remains hyper-

phosphorylated and inactivated by mTOR. Under hypoxia (1% O2), inactivation of mTOR leads to accumulation of hypophos- phorylated 4EBP, which binds and inactivates eIF4E leading to reduced global translation. Hypoxia also induces the expression of HIF-2α, which interacts with RBM4, eIF4A and eIF4E2. Binding of RBM4 to the RNA Hypoxia Response Element (rHRE) allows the recruitment of this complex to a subset of transcripts and their translation by an eIF4E2-dependent mechanism.

plex stimulate global translation under of eIF4E2 prevents this binding. Thus, affinity for the cap structure and, in hypoxia? The cap-binding complex, eIF4E2 tethers the HIF-2α/RBM com- normal conditions, it cannot compete composed of eIF4E, the RNA helicase plex to the cap structure. with eIF4E for cap binding [10]. During eIF4A and the scaffolding protein Importantly, silencing of eIF4E2, hypoxia, however, eIF4E is inactivated eIF4G, is a common target for transla- but not eIF4E, prevented the transla- by 4EBP, increasing the chances that tion regulation. Testing interactions with tion of rHRE-containing targets and eIF4E2 binds the cap. Increased local components of this complex revealed dramatically reduced the amount of 35S- concentration of eIF4E2 on specific that neither HIF-2α nor RBM4 interact methionine incorporation into nascent targets promoted by interactions with with eIF4E or eIF4G, but they do inter- proteins under hypoxia. Conversely, RNA-binding proteins [2], or post- act with eIF4A and the eIF4E isoform silencing of eIF4E, but not eIF4E2, translational modifications of eIF4E2 4E2, suggesting that the HIF-2α/RBM4 inhibited basal translation of HIF-2α [11] may contribute to translational complex binds to an alternative cap- targets and reduced global translation stimulation under particular conditions. binding assembly. eIF4E2, also known under normoxia. These results uncov- Finally, interaction of eIF4E2 with as 4E-homologous protein (4EHP) [6], ered a switch from eIF4E- to eIF4E2- specific initiation factor isoforms could does not interact with eIF4G and binds dependent translation in response to also provide versatility in translational only weakly to 4EBP1, a factor that oxygen tension (Figure 1). They further control. binds and inhibits eIF4E under hypoxia identified eIF4E2 as an activator of The stimulatory role of eIF4E2 in [2, 7-9]. The low affinity of eIF4E2 translation. translation during hypoxia contrasts for 4EBP1 explains its resistance to One question that emerges from this with the known function of this factor inactivation in this metabolic condi- study is why eIF4E2 does not function in other biological contexts. During tion. HIF-2α and RBM4 are retained in in general translation during normoxia. Drosophila embryo axis formation, for m7GTP cap columns under hypoxic, but The explanation may lie on the obser- instance, d4EHP is recruited to caudal not normoxic, conditions and silencing vation that eIF4E2 has a weak binding and nanos mRNAs by the RNA-binding www.cell-research.com | Cell Research npg 1636 proteins Bicoid and Brat, respectively, Acknowledgments BP1. FEBS Lett 2004; 564:58-62. and inhibits the translation of these tran- 8 Joshi B, Cameron A, Jagus R. Char- scripts [12, 13]. The capacity of d4EHP I thank Juan Valcárcel for critical com- acterization of mammalian eIF4E- family members. Eur J Biochem 2004; to inhibit translation has been attributed ments on this manuscript. Work in my lab 271:2189-2203. to its inability to bind eIF4G, which has is supported by BFU2009-08243 and Con- solider CSD2009-00080 from MICINN. 9 Hernández G, Altmann M, Sierra JM, et a primarily scaffolding function during al. Functional analysis of seven genes translation initiation and supports mul- References encoding eight translation initiation tiple interactions for ribosome recruit- factor 4E (eIF4E) isoforms in Droso- phila. Mech Dev 2005; 122:529-543. ment. In agreement with this, human 1 Hernández G, Vazquez-Pianzola P. 10 Zuberek J, Kubacka D, Jablonowska A, eIF4E2 does not interact with eIF4G Functional diversity of the eukaryotic et al. Weak binding affinity of human during hypoxia. How can then eIF4E2 translation initiation factors belong- 4EHP for mRNA cap analogs. RNA ing to eIF4 families. Mech Dev 2005; stimulate translation? One possibility 2007; 13:691-697. 122:865-876. is that a specific eIF4G isoform acts 11 Okumura F, Zou W, Zhang DE. ISG15 2 Uniacke J, Holterman CE, Lachance together with eIF4E2 during hypoxia. modification of the eIF4E cognate G, et al. An oxygen-regulated switch Indeed, precedent for functional eIF4G 4EHP enhances cap structure-binding in the protein synthesis machinery. Na- isoforms exist in the literature. The activity of 4EHP. Genes Dev 2007; ture 2012; 486:126-129. 21:255-260. eIF4G homologue protein DAP5/p97 3 Lin JC, Hsu M, Tarn WY. Cell stress 12 Cho PF, Poulin F, Cho-Park YA, et al. A is known to support translation of select modulates the function of splicing new paradigm for translational control: mRNAs during endoplasmic reticulum regulatory protein RBM4 in translation inhibition via 5′-3′ mRNA tethering by stress and has also been proposed to control. Proc Natl Acad Sci USA 2007; Bicoid and the eIF4E cognate 4EHP. 104:2235-2240. function in non-stressed cells [14, 15]. Cell 2005; 121:411-423. 4 Lin JC, Tarn WY. RNA-binding motif Although DAP5 lacks the eIF4E-bind- 13 Cho PF, Gamberi C, Cho-Park YA, protein 4 translocates to cytoplasmic ing domain, it could retain the capacity Cho-Park IB, Lasko P, Sonenberg N. granules and suppresses translation via Cap-dependent translational inhibition to interact with eIF4E homologues. argonaute2 during muscle cell differen- establishes two opposing morphogen Alternatively, novel proteins could tiation. J Biol Chem 2009; 284:34658- gradients in Drosophila embryos. Curr substitute for the scaffolding function 34665. Biol 2006; 16:2035-2041. of eIF4G during translation initiation. 5 Ray D, Kazan H, Chan ET, et al. Rapid 14 Nousch M, Reed V, Bryson-Richardson α and systematic analysis of the RNA rec- One candidate for this is HIF-2 itself, RJ, Currie PD, Preiss T. The eIF4G- ognition specificities of RNA-binding which strongly interacts with eIF4A. homolog p97 can activate translation proteins. Nat Biotechnol 2009; 27:667- Delineation of the composition of the independent of caspase cleavage. RNA 670. 2007; 13:374-384. cap-binding complex associated with 6 Rom E, Kim HC, Gingras AC, et al. 15 Lewis SM, Cerquozzi S, Graber TE, eIF4E2 during hypoxia may clarify Cloning and characterization of 4EHP, Ungureanu NH, Andrews M, Holcik M. these issues. Nevertheless, the results a novel mammalian eIF4E-related cap- The eIF4G homolog DAP5/p97 sup- of Lee and colleagues illustrate the binding protein. J Biol Chem 1998; ports the translation of select mRNAs 273:13104-13109. plasticity of the translational machinery during endoplasmic reticulum stress. 7 Tee AR, Tee JA, Blenis J. Character- to adapt to changing metabolic condi- Nucleic Acids Res 2008; 36:168-178. tions. izing the interaction of the mammalian eIF4E-related protein 4EHP with 4E-

Cell Research | Vol 22 No 12 | December 2012